get MIME types without strictly requiring python-magic

get MIME types without strictly requiring python-magic

There are two forms of python-magic, one included in libmagic that is
default on Debian, and another Python wrapper for libmagic that is
called 'python-magic' on pypi.  Those both rely on the compiled binary
library libmagic.  For platforms without good package management, fallback
to using the built-in 'mimetypes' library if 'magic' is not available.

This supports OSX, Windows, and BSD.