File extension

A file extension (also filename extension) is a suffix to the name of a computer file applied to indicate the encoding convention (file format) of its contents. In some operating systems (for example Unix) it is optional, while in some others (such as Windows) it is a requirement. Some operating systems limit the length of the extension (such as DOS and OS/2, to three characters) while others (such as Unix) do not. Some operating systems (for example RISC OS) do not use file extensions.