Document Extension being changed

I'm guessing this is a standard feature of Appian, which would rightly treat extensions as all lower-case by default.  What exactly is your use case for needing the "_TEST" snippet capitalized?  If you have some functional requirement that can't be accomplished any other way, I suppose you could open a support case with Appian asking for some sort of behavior change (or override option) to be added to their development backlog, just be aware that you'll probably be waiting a while for it to be implemented (if ever).