Is it literally just a file extension or is it an entirely different data format?
A good program shouldn't be freaking out about just the name of a file. Obviously the file extension is a hint that can be used for filtering what files are shown by default in the file picker or whatever, but if you explicitly give a file to a program and say "open this as a [some format]" then it shouldn't be second-guessing based on the name. For example, I should be able to use a text editor to open a text file called "photo.jpg" if I want to, and any text editor that fails to open that file is garbage.