Clean up some minor UI behaviors in Differential

Summary:
Minor UI tweaks:

• Use the dynamic icon for each file (e.g., image, text), not a hard-coded icon.
• Render the path (less important) in grey and the filename (more important) in black.

Test Plan: {F4966176}

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17957