I think .jpl is more than ok. IMO it is not too similar to JEX, but rather consistent with JEX.
Joplin EXport
Joplin PLugin
If we want to be consistent and not type a short story for an extension, we should go with .jpl
What's underneath is a different story. Whether it's tar, zip, or whatever, it should support symlinks and keep permissions intact. Although maybe permissions are not that important, since we should only need to read those files.