Depending on the Python version, this might raise an error: Before 3.8,
mimetypes.guess_type could not deal with a pathlib.Path, and only
accepted a string. guess_type is called internally by FileResponse, so
to make sure we don't run into errors, we stringify the path before
passing it to FileResponse.

Even though the change is "relatively simple", it does add quite a bit
of stuff since now we do need metadata in the database.