Windows only: When you've got a file with an extension you don't recognise, the free command line tool TrID can help. We've covered how to use the web to find out what kind of file you have, but TrID adds a probability output. Rather than simply listing all the file types that have ever used the extension you're searching for, TrID analyzes the file itself and returns the probability of the file being a certain type, as shown.
Using a Mac or Linux? Check out how to identify files from the terminal. TrID is a free download for Windows only.