Use pathlib to do the filepath manipulations in `File` class

At the moment you have a class `File` that's implementing much of the logic that's found in [Pathlib](https://docs.python.org/3/library/pathlib.html) but in such a way that appears to be not cross platform and a potential source of bugs.

I'd suggest using Pathlib to handle all of the file path manipulations instead and then deal with any of the tiers functionality separately.

Perhaps I'm misunderstanding what `File.setExt` does, but I'm wondering why this code exists?