A filename extension is a
suffix to the
name of a
computer file applied to indicate its
type. It is commonly used to infer information about what sort of data might be stored in the file. The description above is meant to mostly explain the intent of filename extensions: a true definition, giving the criterion for deciding what part of the file name is its extension, belongs to the rules of the specific
filesystem used; most times the extension is the substring which follows the last occurrence, if any, of the
dot character (e.g. "txt" is the extension of the filename "readme.txt", "html" the extension of "mysite.index.html"). On filesystems on mainframe systems such as
MVS,
VMS, and PC systems such as
CP/M and derivative systems such as Microsoft
DOS, the extension is actually a separate namespace from the filename. This is different from
Unix-like operating systems, where filesystems do not actually support the notion of an extension, where a suffix is not a separate namespace, and where even having a suffix is voluntary for executables, as
permissions are used to decide whether a file is executable.
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