Q: What is a File Extension?

A: A file extension, also called a filename extension, is the suffix at the end of a filename, which indicates what kind of file it is. For example, you can tell that the file "computer.jpg" is a JPEG image file, based on the ".jpg" file extension. Similarly, you can tell that "readme.txt" is a text document and "document.docx" is a Microsoft Word document. Knowing what extensions common file types use can be helpful.

While most file extensions consist of three characters, some have fewer (e.g., .H, .DB, .PS) while others have more (e.g., .GADGET, .TORRENT, .TAX2010). When you double-click a file, your computer uses the file's extension to determine what program should open it. If you change a file's extension, it may also change the program your computer uses to open the file. Therefore, you should be careful when changing a file's extension, since your computer may be unable to open the file. For example, if you change a file with a ".txt" extension to a ".doc" extension, Microsoft Word should open it. However, if you change a ".txt" file to a ".jpg" file, the file will not open.

Since there are tens of thousands of software programs available, there are also tens of thousands of filename extensions. While it is helpful to learn the most commonly used file extensions, it is not possible to remember them all. So whenever you come across an extension you don't know, visit FileInfo.com to find out what kind of file it is.

Updated: June 13, 2011