Hi

A 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 DOS) 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 filename extensions. UNIX accepts the separator dot as a legal character but does not give it a special recognition on the OS level.

Thanks