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.

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.