Have you ever been trying to choose a new file extension? Well, look no longer...
A gem website I found a while ago that seemed to take ages to find was the following: http://www.fileinfo.com/
It keeps up-to-date records of not just existing file extensions but also what programs use them and descriptions of the file types and how common they are. This means you can decide whether to use the same file extension if it's rare or search different file extensions till you find one that doesn't exist. :)
Happy choosing! ;P