Good idea, i will maybe rewrite my tool with a full support of these extensions.
Then again, it may just be my personal opinion that three-letter extensions are best, but it might be wise to stick with them just in case.
Also, two very popular games occupy the .per file extension, which are Age of Empires/Rise of Rome, and Age of Empires II (Age of Kings)/The Conquerors. These games use .per files as personality files to tag with artificial intelligence files (.ai) that they use. So, I wouldn't advise using it.
Again, these things are just what I think. You're welcome to keep or change them—its ultimately up to you of course. :D
Yes, that's why i suggest more search for the two first extensions