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