Add Filesystem::getMimeType()

Summary: If Fileinfo is avilable, use that instead. If file isn't present, try
mime_content_type().

Test Plan:

• Ran mime.php on a bunch of files on a couple of systems.
• Commented out blocks to force each block to execute.

Reviewers: btrahan, davidreuss, Koolvin

Reviewed By: davidreuss

CC: aran, epriestley

Maniphest Tasks: T869

Differential Revision: https://secure.phabricator.com/D1609