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Guide to Open Unknown File types and Repair File Extension Errors
Step-by-Step Instructions
Updated November 17th, 2008

Tip: Click here for a list of the top 20 File Extensions or search 1000+ file extensions in the Filetonic File Extensions library


Table of Contents

Introduction - Determining Unknown File Types
How To Open Unknown File Types

     Associating Extensions With Applications
     Spam, Viruses, And Malware
     The Dangers Of Digital Infection
     Third Party Preview Software
     Top 50 File Extensions And The Associated Programs
Summary And Recommended Action

Introduction - Determining Unknown File Types

The file extension itself is commonly the best guide there is to help determine the appropriate program to open that file.

The extension itself is normally three characters long, although this isn't necessarily the case. Because there is a very finite limit to the number of three letter combinations it is not unusual for one extension to be used by more than one file type. It is also becoming more common to see file extensions using four characters in order to alleviate some of the possible confusion.
Regardless of the file extension, though, every file type needs to be opened by an application that is compatible with that extension. While some are fairly common knowledge, such as Microsoft Office documents, others can be less obvious and less apparent. Similarly, some types of file can be opened with a number of different applications (many audio and image files, for example) while others will work only with one application.
The Windows Registry contains settings that determine the application used to open extensions of each type. Once set this means that whenever you attempt to open a file of a specific type your Windows operating system will use that same application. This provides a quick and convenient method of opening files and negates the need to select an application whenever you open a file.
Because of the number of different file types that are used it is fairly commonplace to receive or acquire files with an unknown file type. In most cases users need to find the appropriate application before the file can be opened, but certain precautions should always be taken before attempting to open any unknown file types.
If a file of an unknown type has been emailed or delivered to you by a trusted source then typically the file should be safe. On the other hand, if you have downloaded the file from the Internet, especially using peer-to-peer file sharing services, then there is a very real possibility that the file is, in fact, a spyware or virus ridden. Opening these files may lead to your computer becoming infected with malware and malicious software that is difficult to remove and can cause errors with your system or can lead to the theft of your personal information.
This guide looks at the many aspects of unknown file types. This includes typical steps to open or repair unknown files as well as a guide to the top 100 most common file extensions with associated programs and some notes. We also aim to help users determine whether an unknown file type could, in fact, be an undesirable application that should be deleted rather than opened.

How To Open Unknown File Types

In order to open any file you must have software that is applicable with that file type. This software is able to decipher the code and display or execute the resulting file in the appropriate manner.

This means that without the proper application it is typically impossible to work with the file as you would with a recognized file type.
It may be possible that certain file types can be viewed or previewed with certain applications but can only be edited with other programs. Again, this isn't uncommon, and depending on the action you need to take with that file you will need to get the appropriate application. Image files are a common example of this. Applications such as the Windows Picture and Fax Viewer can be used to adequately view the images but in order to edit the file you will need to use another application such as Paint.
As well as using the application that first created the file it may be possible to use a file pre-viewer to preview the contents. In some cases this can even be done using the text editor that is installed on your computer already. Whether you use WordPad or any other text editor it may be possible to see the contents of a file using this method. Simple right click on the file and select Open With and then click Browse to browse your computer and find the text editor you want to use.
Third party previewing software can also be used. These file viewers obviously do not offer the capability to work with or edit the file in question but they do provide a good snapshot of the file contents. Using this preview you can determine whether you want to view and edit the file fully, in which you will need to purchase or acquire the appropriate program, or if the preview is enough.
The first step in opening an unknown file type is obviously to determine the software you will need. Later in this guide is a list of common file extensions and the application or applications that can be used to view or edit these files. With literally thousands of different extensions it is not possible to include every single one in this guide, but those that are not included can usually be found by searching online using the extension as the search phrase.
Once you have found the application you will need to purchase, download, and install it to your computer and then open the file. Depending on the application you may be prompted when installing the program to determine the file types you wish to open with this software. Alternatively, when you attempt to open a relevant file extension and are prompted to decide on the software to use you should tick the box to open all files of this type with that software. In the future when you attempt to open files of this type your Windows operating system will know the application to use in the future.
Every file type is structurally different. Without using the proper application to open a file the architecture of the file can not be read and the file can not be viewed or edited properly. For this reason, the biggest part of the challenge of opening an unknown file type, is finding the proper software that is most closely and regularly associated with that file structure.
Some file types can be converted to another format for opening with an application you have. For example a 123 file is a Lotus spreadsheet file but using a conversion tool it is possible to open the file in Microsoft Excel in order that you can view and edit the contents using Excel instead. This also negates the need to purchase a copy of the Lotus 123 software.

Associating Extensions With Applications

The Windows registry contains a number of entries that are associated with the opening of each file type present on your computer.

Associating a file extension with a specific application means that the Windows Operating System will consult the Registry and determine that the associated program should be used to open that type of file. Windows will then automatically use this application in order to open files of this type in the future.
Associating applications with extensions can save a good deal of time in the long run. Instead of having to manually select the application to use whenever you attempt to open a specific type of file, Windows can automatically use the same application each time. When you first open a file type that Windows does not recognize you will be asked to determine which application should be used. On this same dialog window there is also a checkbox that indicates Windows should use this application to open all files of the same type. If you click this box then your system will add a registry entry so that this application becomes the default application for opening files with that extension.
Alternatively, you can change an association for any file type in the following way:
>  Open Windows Explorer and highlight a file with the type you wish to change. It may be necessary to alter your system settings so that you can view the file extensions (this is explained in the next section of the guide).
>  Right Click the highlighted file and select Open With...
>  Click Browse and select the  program you wish to use
>  Highlight the check box next to the box that reads “always use the selected program to open this kind of file” and click OK
By Default most Windows computers are not set to show the extension of every file. This information can be vital, especially when opening new files and attempting to determine the best course of action with a new file type. In order to view the extension type of files in Windows follow this procedure:
>  Open Windows Explorer
>  Click Tools and select Folder Options
>  Select the View tab
>  Remove the tick from the “Hide extensions for known file types” box

Spam, Viruses, And Malware
It is inevitable that the majority of computer users will come across new file types on a fairly regular basis. Obviously, the more files you receive, the more likely it is that this will happen.

It is important to realize the risks before opening files that are delivered by email or that you download from the Internet. While the majority of files may be safe to open and use it would be naďve to believe that this will always be the case so some care must be taken in these instances.
Care should always be taken when opening email extensions from unknown senders. A good rule of thumb is to completely ignore them without a valid reason for being sent that can be checked and verified before opening the file. Unfortunately, certain viruses or worms are spread using the contact email address book of infected computers. This means that spurious looking emails with unwarranted attachments even from people you do know should also be treated with extreme caution.
The fact is that viruses and other malicious software and code can be spread using a huge variety of file extensions. Vigilance should be concentrated more on the sender of the email and the content of the email rather than the type of file that has been delivered. Where it is possible to preview the file using third party software this may provide a safer alternative to opening the file properly and fully, although again some viruses can still be contracted in this way.
Always ensure that you can view file types of attachments and other files before opening them. Look for misleading file names as being a very good sign of possible infection. For example, a virus may be named pictures.mpg but in actual fact be a .exe file. By showing file extensions this would be displayed as pictures.mpg.exe – this should immediately ring alarm bells and prevent you from opening the file.

Regular Internet users

Those of us that use the Internet on a frequent basis are typically at more risk than light and infrequent surfers.

Leaving your email address on web pages and sites inevitably leads to the receipt of spam and junk email but it can also lead to the receipt of more sinister emails.

Hackers and malicious software coders use automated software to lift email addresses from the wilds of the Internet and then forward viruses and other undesirable attachments to your email.
As well as emails, though, other threats also exist. Peer to peer file sharing has become increasingly popular and with this increase in popularity has come an increase in the number of infected files that are forwarded using this method. Never download unrecognized files and always ensure that any file you do download has the kind of properties that you would expect. If you can't view the file type or a file has a different file type to the one you would expect then you should avoid it.
Chat rooms and Instant messenger can also be used to transmit viruses over the Internet. You are the most potent weapon you have against viruses and other online threats so always be diligent when working with unknown file types from any sender.

Third Party Preview Software

Even genuine file types can cause problems. Namely, if you do not have the appropriate application to open a file then it is highly likely that you will struggle to open the file and view its contents never mind edit it. In some cases, though, it can be possible to use third party applications to view or preview files. This is especially true with media files such as images or movies.
Windows Media Player can normally be used to view images and videos and listen to audio. However, should you wish to change the contents of the files then you will need to have the appropriate application installed. In order to help out we have listed 50 of some of the more popular and common file extensions as well as details regarding the file type and how you can view and edit the files.

Top 50 File Extensions And Their Associated Programs
This is a list of some of the more popular program extensions as well as the applications that are most commonly used to view or edit the files. Short notes are also included regarding any other methods that can be used to view or preview the content of the files before fully opening them. While this list is not a complete list of every single extension type available it is an extensive list that includes the vast majority of popular extension types that you are most likely to come across.
File Extension .doc
The most common instance of the .doc file extension is Microsoft Office Word documents. Word is the word processor that is included in the Office suite of tools. A number of programs can be used to view and even edit .doc documents. As well as Word itself, there are third party Microsoft viewers and some free and open source word processors will also enable you to edit all contents of the document in the same way as you would using Word itself.
File Extension .log
Files with the extension of .log are, unsurprisingly, log files. These are used by various different applications and the name of the file is usually indicative of exactly which application created the log. It may be possible to view the contents of the log file using any standard text editor or you can use the application that it is associated with.
File Extension .msg
These are email messages created by any of a number of email applications. Most commonly .msg messages are associated with Microsoft Exchange Server. Some Microsoft viewing applications may enable you to view the contents of the file but you will typically need the original application in order to edit its contents.
File Extension .rtf
An RTF file is a Rich Text Format file and is one of the more common types of basic text file. These are also used as Microsoft help files and viewing is possible using any application that enables you to view text files. This includes Microsoft Word or any text editor.
File Extension .txt
A txt file is a basic text file used by a lot of applications in a lot of circumstances. In order to view and edit a file with a .txt extension users can use Microsoft Word or any application deemed to be a standard text editor.
File Extension .wpd
There are a number of possibilities with regards to .wpd files. The 602PC Office Suite, an alternative to the Microsoft Office suite, uses .wpd as an extension for any documents created with any of the four applications. A wpd file may also be an ACT Word Processing Document, a Window Works Document, or a Windows Printer Driver. The location of the file should be indicative of which is most likely to be the case.
File Extension .wps
The most likely instance of a wps file is as a Works Text Document. More recent versions of Microsoft Word will open some Works documents. If this is not the case for you then you may need to use a converter that will convert the .wps file into a .doc or other word processor file.
File Extension .123
A .123 file is a document created by the Lotus 1-2-3 Spreadsheet application. The application is no longer supported but some people do still use this spreadsheet. If you can not obtain a copy of this spreadsheet application then you may want to consider using a conversion tool.
File Extension .csv
A csv file, shor tfor Comma Seperated Variables and also referred to as a Comma Delimited File, is essentially a database of information that consists of individual records separated on individual lines and each record separated by a comma. While many people consider this to be an Excel format it is not actually associated with Microsoft except that Excel is the most common application used in order to open .csv files. Typically, though, a .csv file can be opened using any spreadsheet or word processor document.
File Extension .dat
This extension is used by a lot of different applications that are quite different in their use and architecture. As such, finding the appropriate one is essential for opening these files. One common use of the .dat extension is for Internet cache files.
File Extension .db
The .db extension is an extension used by many database applications or to signify that the file is a database required by an application. Some of the more common applications to open and edit .db files include the Oracle database, Windows ThumbNail Database, ArcView Object Database, and Progress Database.
However, it may also have been created by the Microsoft Works database application. These applications are quite different in many aspects and, as such, it is highly unlikely that you will be able to open the file unless you have the appropriate database application installed.
File Extension .dll
A dll or Dynamic Link Library is a short script or command that is stored within the library of your system. Whenever an application requires the use of this command it is called and the feature or action that it is associated with is performed. As such there is a large number of dll files to be found within computers running certain versions of Windows and, unless the file is found to be damaged, broken, or false, it should be left to run as is.
File Extension .mdb
An mdb file is a Microsoft Access Database file that can be viewed using Access itself as well as Microsoft Viewers. Alternatively, if you need to edit the file, then you should use the Microsoft Access application or another application that supports and recognizes Access database file structures.
File Extension .pps
The most likely instance of a pps file is a PowerPoint Slide Show. This type of file requires either the use of Microsoft PowerPoint or a similar application that recognizes the same file types (the open source Open Office is a commonly used application).
File Extension .ppt
A ppt file is usually a PowerPoint Presentation. As well as being able to use Microsoft PowerPoint to view and edit these files it is also possible to use other applications such as Open Office that offer Microsoft compatibility.
File Extension .sql
SQL, or Structured Query Language Data, is a query based database. A number of different applications can be used to alter SQL databases. While SQL databases are commonly used for containing data online they are one of the oldest used types of database available.
File Extension .wks
A wks file is a spreadsheet file that may have been created either by Lotus 1-2-3 or by Works for Windows. Excel by Microsoft will open and save to the 1-2-3 version of this file and also in versions of Works 2.0 and earlier. If you need to open a .wks file then Excel is likely to be your best bet to opening and editing the file.
File Extension .xls
An xls file is a spreadsheet or worksheet created by either of Microsoft's Excel or Works spreadsheet applications. The Microsoft Viewer can be used to view the file if it is an Excel spreadsheet, otherwise it is possible to use the Excel application or any other compatible spreadsheet application to view and edit these files. With Works for Windows xls files it is necessary to have a relevant version of Works installed on your computer.
File Extension .xml
The xml, or extensible Mark-up Language, file is a general markup language page most commonly associated with web programming. Specifically it was designed as a standard to replace html although html does remain more popular than its replacement. Xml files can be opened using any standard text editor and most word processor applications.

File Extension .3dm

.3dm are 3D model files that could have been created using one of several different 3D modeling applications. There are several such applications including Cadent, IN3D, QuickDraw, and Rhine 3D. Typically, any of these applications can be used to open, view, and edit 3dm files.

File Extension .3dmf

A 3dmf file is a QuickDraw meta file. QuickDraw is a 3D modeling application produced by Apple Computer Inc. The QuickDraw application is required to edit this type of file, however, there is also a 3DMF viewer that can be used to quickly view or preview the file to determine whether it is a necessary file and whether you need to obtain a copy of QuickDraw.
File Extension .ai
Among the several known and recorded instances of .ai files is a vector graphic file created by the Adobe Illustrator application. These files can only be edited using the Illustrator application, however, XnView is a third party application that enables users to view or preview the contents of the file. Doing so will enable you to determine whether the file is necessary and required, and whether you need to install Illustrator in order to properly edit and amend the file.
File Extension .bmp
the .bmp file extension is one of the most common graphic file extensions. Bitmap graphics files are created and used by a huge number of different applications, each editor being able to open a .bmp file and edit its contents. As such, it is possible to use virtually any image editing software in order to view or edit a .bmp file you may have received.
File Extension .drw
It is highly likely that the .drw file you have received is a  graphics file and is most commonly associated with a large number of graphics editing and creation applications. Any of these applications can be used to create, view, or edit these files. If you have any graphics applications already installed on your computer then you should attempt to use this application first in order to open the file.
File Extension .eps
There are two likely uses of the .eps file extension, both of which are types of graphics files. One is a vector graphics file and can be opened with any working vector graphics editor such as Adobe Illustrator. The other is a photo file which can, again, be opened by applications like Illustrator for complete editing and viewing of the file.
File Extension .gif
a gif, short for graphics interchange format that is used by a lot of programs and applications as well as displayed in a number of different publications. The good news with this file type is that it can also be opened and edited using a wide range of graphics software such as the GIMP, which is an open source application. There are also free viewers that can be used to view the contents of the file although do not allow for any editing of the file.
File Extension .indd
A indd file is a document created by Adobe InDesign CS3. This is a design application that is used to create good looking documents and publish them to any of a number of different media formats. This is the only application that can be used to view and edit .indd files so it will be necessary to purchase and install a version of this software to view the file. A free trial of the software is currently available on the Adobe website providing a means to view files created with this software.
File Extension .jpeg and File Extension .jpg
JPEG files are most commonly used for digital photograph files and also for publishing images to the Internet. The file type is not ideal for directly opening, editing, and resaving files and instead should be opened as a different file type and then saved as a jpeg file type for publishing the file. .jpeg files can, however, be opened and edited with a variety of graphics editing applications and viewed using even more applications.
File Extension .mng
MNG is short for Multi-Image Network Graphic Animation and is actually a collection of individual visual PNG components. It may also be an Animation created using the Animation Shop software published by Corel. Some viewers may be able to view or preview these files but you will need the appropriate application in order to fully edit them.
File Extension .pdf
A pdf file, short for Portable Document Format, can be viewed using the free Adobe viewer on the Adobe website. There are other PDF viewers also available on the Internet; a number of these applications can be freely downloaded and used in order to view any PDF files. Virtually any file can be converted into a PDF file but PDF editing software will be required to alter the contents of the file.
File Extension .ps
A .ps file is a type of Page Description Language. It is similar in use to PDF and is also created by Adobe. A .ps file is essentially an image of a document and, as such, it can typically be viewed by iamge viewing software. There are a number of free applications, such as XNView, that can open and view these files.
File Extension .psd
The file format .psd belongs to graphics files that are created using the Adobe Photoshop application. As well as being able to open them using the Photoshop program it is also possible to use one of many different standard image editors. The .psd file type is so popular because as well as being compatible with Windows computers they can also be read by Macintosh based PCs.
File Extension .psp
A .psp file type is a graphics file created by the popular Paint Shop Pro application from Corel. Even though this file type is specifically created with this application, psp files can be read using a number of free and commercial image editing applications. If you only need to view the file, rather than edit it's content, then you can use an image viewer instead.
File Extension .qxd and .File Extension qxp
.qxd files are desktop publishing files created by version 5 or earlier of the popular QuarkXpress application. Users require the QuarkXpress application to view or edit these proprietary file types. The .qxp file type is also created by QuarkXpress and also requires the QuarkXpress application to edit these files.
File Extension .tif
TIFF stands for Tagged Image File Format and is a graphics file. It is one of the more popular file types for archiving images because it requires very little space to save a reasonable version of an image. .tif files can be viewed using a huge range of different image viewing applications including the Windows Picture and Fax viewer, which comes as standard with the Windows XP operating system.
File Extension .mp3
An mp3 file is a compressed audio file that has become particularly popular in recent years because of its compatibility with personal audio equipment, computer equipment, and other stereo equipment. Although it depends largely on the bit rate of the file mp3 tracks are typically almost as high in quality as CD music.
File Extension .wav
The .wav file type is a high quality, but consequently large, audio file type. Short for WAVE file the wav file type is commonly used to save audio of a CD quality. The only real drawback of the .wav file is the size of the saved file when compared to other formats, such as mp3. Microsoft Media Player and any other media player will open and play .wav files with no problems.
File Extension .wma
The .wma file type, or Windows Media Audio file, is a proprietary file type that is similar to other audio files but has been compressed using Windows compression techniques. The Windows Media Player and a small selection of MP3 players can open and play wma files, but the iPod and even other popular personal audio players do not play this file type.
File Extension .asf
An .asf, or Advanced Streaming Format, file is a file that creates audio and/or video compressed with codecs. Many media players and similar applications can read these file types. Users should attempt to use any media player they have installed in order to open this file. Typically, Windows Media Player is a good option.
File Extension .avi
.avi is a well used and easily read file type. Short for Audio Video Interleave it is an audio or video file that has been compressed with third party codecs. Many media players can open and play these files including Windows Media Player so playback should not cause any difficulty in the majority of cases.
File Extension .mov
The .mov file type is, most typically, a QuickTime movie file. Usually these are relatively short movie clips designed for ease, speed, and a shorter file size. Files can be viewed using any of an array of media players including Windows Media Player. Other free media players can also be used to play .mov clips.
File Extension .mp4
.mp4 files are movie files that can typically be played using a wide variety of different media players, although it may be necessary to down load the appropriate Codecs from the media player website. Windows Media Player, as well as various other freely available media players can be used for mpeg-4 playback.
File Extension .mpg
An mpg file can be video, audio, or animated clips. Playable with any media player it may be necessary with some players to download the necessary Codecs from the software manufacturer's website. As well as Windows Media Player there are various free and commercial media players that can be used to play this type of file.
File Extension .swf
.swf files are flash files and can range from small buttons on websites to large animations. More and more websites are using flash animation to help liven up otherwise lifeless pages, and as Internet connections become quicker and the technology behind the use of the Internet becomes more complex it is increasingly likely that more web pages will introduce at least a small amount of flash elements. .swf files can be viewed in most browsers or using Flash software and Flash file viewers.
File Extension .wmv
The wmv file type, short for Windows Media Video, is a high quality video format that can be viewed using a good range of media players. The Windows Media Player is the most obvious and accessible option for viewing these files.
File Extension .fnt
The .fnt file type, although mostly replaced by ttf and otf files, is a font file. These files are used by Microsoft Windows and can contain a single true type or vector font. The fonts can be edited using font editing software such as Vsoft and can be viewed using Microsoft applications such as Word.
File Extension .exe
A .exe file is an executable or program file. These file types are not usually associated with any application and double clicking or opening the file will usually cause the application itself to run. .exe files can also be called by other applications in order to perform specific tasks.
File Extension .ws
A .ws file is a Windows Script file. Usually this type of file contains Jscript, and other scripting elenents including XML. Some care should always be taken when opening .ws files because they can contain viruses and malicious code. Special care should be taken if you receive a ws file as an email attachment or via messenger or file download.
File Extension .rar
.rar files are compressed files, or groups of files, that have been compressed using RAR compression. RAR is a more powerful and more effective method of compression when compared to ZIP compression. WinRAR can be used to zip and unzip RAR files on Microsoft Windows computers, while Mac and Windows users can use the Stuffit Expander suite.
File Extension .zip
Despite being less powerful than RAR compression, ZIP compression remains the most popular form of compressing files and folders. A .zip file is the result of this compression and can be opened or created using the majority of compression utilities including WinZip and Stuffit.

Summary And Recommended Steps

Users receive unknown file types from a variety of people, using different sources, and for any of a number of reasons. Some are genuine while others are less so. Viruses can be easily and quickly spread via email as attachments but also through messenger software and through peer to peer file sharing networks. For the most part it is down to the diligence of the individual to ensure that they do not do anything that might put them at greater risk of downloading viruses and other online threats.
Genuine files of an unknown type can still cause problems. Finding the appropriate software to either preview or edit the file in question is essential. While some file types may necessitate the use of just one piece of software for opening and editing, other files can be associated with a vast number of different applications; some free and some commercial.
If you have determined that a file is genuine and being sent to you for genuine purposes then you will need to find the right software. Usually, you should be able to find this out from the person or site that you have received the file from. However, if this isn't an option then you will need to do some research into the file extension yourself. The list earlier in this guide includes 50 of the most popular extensions, but there are literally thousands of different ones currently being used and that number is always increasing. The Internet is a vast source of information and will usually give you the information you require with a simple and quick search on the search engines.
If you want to ensure that you genuinely require the file then you might want to consider the use of third party file previewing software. Preview software can be used to view an image of files without specifically having to open the source itself. This can also help to prevent infection by viruses, malware, and other threats although some will become active simply by previewing them.
Set Windows so that Windows Explorer displays the file type of previously unknown files. Once you have done this and determined the application that is required to open these files you should also set Windows so that it uses the same application whenever you attempt to open files of that type. This process can save time and effort in the long run because Windows will automatically use that application without requesting confirmation and without requesting that you select an application every time.
Diligence is vital. Anti virus, anti spyware, and other digital security software can help to prevent the infection of your system with the majority of viruses and malware. However, some infections can still make it past this level of defense. In these cases you are literally the only protection you have against these threats.

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