Hmm, you could check for all the common picture, video and audio headers and try to extract all other files with unknown header instead...

Tbh don't know if there's any good solution as long as the dev works actively against modders.
Oh, btw. I heard
Big City Pleasures gets an update soon. The game comes with a custom rpa format (PEA files) which does more than changing the header/extension. (compression was changed from zlib to bz2 and some offsets are different)
I made a custom rpatool version back then, which can be found
here to unpack those, just FYI if some reports come up once the game is updated... ^^