Assuming you mean "filetype" or "extension" in stead of "format", all I can do is point you to http://filext.com/file-extension/file. That says it's a file without extension. So it can be anything.

How to use a program (that's what you ask): see the user manual or the help.
How to use a file: open it with the right program. If you made it yourself, you'll probably know. If someone else made it for you, ask him.

Kees