This adds a method for checking whether a mimetype is supported or not.

This is useful for code that deals with uploading binary files, to help

decide which files should be uploaded. We have similar logic for this in

RBTools. Valid and invalid mimetypes are saved to the cache and are

checked against when available, which should help speed things up when

the caller is checking a sequence of mimetypes.
This change also fixes a fault with guess_mimetype. The function runs

the file command in a subprocess to determine the given file's mimetype.

The function doesn't properly close and clean up the subprocess's resources

(the file descriptors for stdin and stdout). In the Review Board server

process which runs for a long time, we're relying on Python's garbage

collector to close everything. The garbage collector is not deterministic

and it could take a while before the resources are cleaned up. It's better

to deal with these ourselves to avoid any unexpected side effects. This

change makes sure the resources get cleaned up when they're no longer used.

Ran unit tests.
Used in the Bitbucket Server Pull Request integration code in Power

  Pack.
Saw the ResourceWarning: unclosed file coming from guess_mimetype

  go away in the logs.