I checked out your edit and what’s going on is pretty weird. my recommendation (if you don’t wanna just use the native import/export feature and give up on DB dumps) is to (on the new server) create a minimal tandoor docker-compose.yml, run it, do minimal setup, see if it works, and if the fresh container works, check the permissions, then just replicate those perms for your other postgres directory that contains data you actually care about. good luck!
fair but “it” here is pretty vague. my guess is that, in your case, whatever part of “it” that processes the image handles webp files just fine but for some reason the devs made “it” reject file extensions that don’t match “.jpeg”, “.png”, “.whatever_else” for some reason before the file gets handled further.