I found a nice website with a large file extension list, and all the details that go with each extension. It is:
It has easy-to-understand description and a legibly given list of associated applications. You can quickly search by extension, or view common and most popular file extensions.
You can also check out the extension categories such as audio and music files, multimedia, graphic and image file types. This makes it a great place to discover new software that are associated with the extension category you are interested in.