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

The file extension 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.

It may be possible to repair the file extension associations in the Windows Registry.

RegCure offers you the ability to detect and repair corrupt registry associations that may lead to file extensions not opening or unknown file types that do not correctly identify the associated program. Click the green highlighted link above to scan and repair unknown file types right now!

Continue to the next section titled How to Open Unknown File Types