Online Computer Dictionary

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Queried for: Filename extension  About Filename extension

Definition:

The portion of a filename, following the final point, which indicates the kind of data stored in the file - the file type.

Many operating systems use filename extensions, e.g. Unix, VMS, MS-DOS, Microsoft Windows. They are usually from one to three letters (some sad old OSes support no more than three). Examples include "c" for C source code, "ps" for PostScript, "txt" for arbitrary text.

NEXTSTEP and its descendants also use extensions on directories for a similar purpose.

Apart from informing the user what type of content the file holds, filename extensions are typically used to decide which program to launch when a file is "run", e.g. by double-clicking it in a GUI file browser. They are also used by Unix's make to determine how to build one kind of file from another.

Compare: MIME type.

{Tony Warr's comprehensive list (http://camalott.com/~rebma/filex.html.

{FAQS.org Graphics formats (http://faqs.org/faqs/graphics/fileformats-faq/.