There are numerous file extensions, many of which we come across every day - htm, html, zip, rar, php, - well there's too many to list here, but what do you do if you come across a file with an extension unknown to you?
Hmm, a file extension library would be handy!
Go there and you'll find a hyperlinked list of common and not so common file extensions.
Have a look for the extension you're trying to identify, click on it and you'll find information about the extension, what to do if windows can't open your file and, when you scroll down the page, more detailed information about the extension.
Check out File Extension Library.