How to create file extensions longer than three characters
Not all early operating systems supported file extensions longer than three characters. Below lists operating systems and their support for long file extensions. If your operating system supports long file names, renaming a file or creating a new file with long file extensions is possible.
Microsoft DOS, Windows 3.x, Windows 95, Windows 98, and Windows NT
All versions of MS-DOS and these versions of Microsoft Windows do not support file extensions longer than three characters.
Microsoft Windows 2000, Me, XP, Vista, 7, and 8
These versions of Microsoft Windows do support file extensions that are longer than three characters. However, it is important to realize that most Microsoft programs that rely on file extensions commonly only utilize three character file extensions. Therefore, to use an extension longer than three characters, make sure the program supports the extension.
BSD, all Apple Operation systems, Linux, OS/2, Unix, and variants
Each of these operating systems support the availability of a file extension that is longer than three characters.
Even though a file extension longer than three characters is supported in these operating systems, the programs used to open the files must also support long file extensions.