Mimetype guessing was returning the mimetype as `bytes` rather than `str` (`unicode`), due to the default behavior of `subprocess.Popen`. On Python 2 this is fine because bytes and unicode are pretty freely intercompatible, but on Python 3 this would then explode later on when attempting to serialize a file attachment object with that mimetype set, since `parse_mimetype` didn't understand `bytes`. While I was here I fixed the order of arguments in a log message to make more sense. Testing Done: I had been experiencing problems when using certain Review Bot tools with the Review Board server running on Python 3.8. When the tool attempted to create the `tool-output` attachment, it would get an error serializing "bytes" and spit out a warning about being unable to parse the mimetype. After this change, everything works as expected.