Epic Megagames is a **** company, they used to change all their file extensions to proprietary formats for their games, which they didn't even take advantage of in order to port their software to other operating systems or consoles.

Changing your file extensions is NEVER NECESSARY. Computers don't use this information when opening a file. This information is only used to sort and select files when opening them in an already-installed application. Hence the form for file formats in any "Open file..." dialog.