RBTools 5.1 introduced new support for filename-based mimetype guessing

using puremagic. This made guess_mimetype keyword-only, but we still

had a call site that was passing in positional arguments, causing it to

crash when uploading binary files.
This fixes up the call site, and changes guess_mimetype to go through

the standard function signature deprecation process so call sites can

continue to pass in positional arguments with a deprecation warning.

Unit tests pass.