file extensionA File Extension is the suffix part of a filename which labels the file type. For example, space.gif has a file extension of .gif, this extension labels the file as a GIF graphic file.
File extensions are extremely important, especially within the Internet and Web environment. File extensions instruct browsers, FTP clients, and other applications as to how to handle, manipulate, or use a file. Care needs to be taken so that the correct file extension is used when naming, renaming, moving, or copying files. Without correct file extensions, applications may not be able to properly access or process a file.
Most file extensions are only three characters long, however there is no minimum or maximum length for a file extension. The only limitation to the length of an extension (and a full file name) is determined by the overall maximum number of characters which can be used to name a file. In DOS--up to and including MS-DOS version 6.23--filenames are limited to 11 characters, a maximum of three which can be used as the file extension. On Mac, UNIX, and Win95/NT systems, the filename limitation is 256 characters with no limitation on how many of these 256 characters can be used for one or more extensions. Even when you are able to use extensions of any length, it is customary to stick with three letter extensions.
It is not uncommon to find multiple extensions on archives files. For example, Macintosh archives can have the extension .sea.hqx, which reveals that this file has been compressed with two different types of archival utilities. UNIX archives can have the extension .tar.Z, meaning the file has been tar'ed and Z compressed.