<filename extension> The portion of a filename, following the final
point, which indicates the kind of data stored in the file.
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
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.
FAQS.org Graphics formats.
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