>Most extensions have three characters, which means the search space is pretty crowded. You may want to consider using four letters.

Is there a reason not to use a lot more characters? If your application's name is MustacheMingle, call the file foo.mustachemingle instead of foo.mumi?

This will decrease the probability of collision to almost zero. I am unaware of any operating systems that don't allow it, and it will be 100% clear to the user which application the file belongs to.

It will be less aesthetically pleasing than a shorter extension, but that's probably mainly a matter of habit. We're just not used to longer file name extensions.

Any reason why this is a bad idea?