Filename extension
A filename extension is an identifier specified as a suffix to the name of a computer file, often separated from the filename with a dot, or other character, that indicates a characteristic of the file contents or its intended use.
Some file systems implement filename extensions as a feature of the file system itself, and may limit the length and format of the extension, while others treat filename extensions simply as a part of the filename without distinction.
Usage
Filename extensions may be considered a type of metadata. They are commonly used to imply information about the way data might be stored in the file. The exact definition, giving the criteria for deciding what part of the file name is its extension, belongs to the rules of the specific filesystem used; usually the extension is the substring which follows the last occurrence, if any, of the dot character (example: txt
is the extension of the filename readme.txt
, and html
the extension of mysite.index.html
).
On file systems of mainframe systems such as MVS, VMS, and PC systems such as CP/M and derivative systems such as MS-DOS, the extension is a separate namespace from the filename. Under Microsoft's DOS and Windows, extensions such as EXE
, COM
or BAT
indicate that a file is a program executable.