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
, 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
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/).