Looks like filext.com didn't catch that one. But with only 46,656 possibilities (if you confine yourself to 3 characters A-Z, 0-9, which I guess you don't have to any more) there are bound to be some collisions in file extensions with all the zillions of programs and data formats that are out there. MP2, for example, is sometimes used for MPEG files (I think) and sometimes for Heroes of Might and Magic 2 map files. The most overused extension might be ".map" which is used for at least 31 different file formats!
Think I should switch to using .SGDK2 as a file extension?