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filename extension The portion of a filename, following the
final point, which indicates the kind of data stored in the
one to three letters (some sad old OSes support no more than
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
used by Unix
to determine how to build one kind of
file from another.