ArcanistChmodLinter should not allow certain MIME types to be executable

Summary: Fixes T5466. An image is an example of a binary which should not be executable. Modify the ArcanistChmodLinter to disallow certain blacklisted MIME types from being executable.

Test Plan: Created an executable image file and ran arc lint over this file.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: richardvanvelzen, epriestley, Korvin

Maniphest Tasks: T5466

Differential Revision: https://secure.phabricator.com/D9723