Filename extension
<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 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://www.faqs.org/faqs/graphics/fileformats-faq/).
