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File Extension printable version
A file extension is the suffix at the end of a filename indicating what type of file it is. For example, in the filename "myreport.txt," the part after the dot ("txt") is the file extension. It indicates the file is a text document. Some other examples include "Document1.doc," which is a Microsoft Word document, and "Image.psd," which is a Photoshop document.

While most file extensions are three characters in length, they can be anywhere from one to five characters long. The extension tells the computer's operating system what program it should use to open the file. It also helps the user see what kind of file a certain document is by just looking at the filename. Both Windows and Mac OS X allow users to change file extensions, which can change the program the computer uses to open the file. While this may work for some files, it can also cause the file to not open at all. 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 .psd file, Photoshop will not recognize the file type.

Because there are literally tens of thousands of file extensions, it is impossible to remember all of them. However, it is helpful to know some of the more common ones, such as .txt, .jpg, .gif, .zip, etc.

For definitions of thousands of different file extensions, visit FileInfo.net.
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Software Terms Updated 10/19/07

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