If you come upon a file extension you don't know, the endlessly useful Wolfram Alpha has a tool built-in that will tell you what it is and what programs you'll need to open it.
While you could use something like Fileext.com, Wolfram Alpha is dead simple and probably something you already have in your custom search engines (and if you don't have it there, you should). Just head to Wolfram Alpha, type in the file extension you're looking for with a dot preceding it (e.g.
.jpg), and search. It will tell you what type of file it corresponds to, as well as give you some sample applications that you can use to open it. Hit the link to read more, including a trick on finding out how to open files that don't have an extension.
How to Open Files that have Unknown Extensions [Digital Inspiration]