Prettier file embeds

Summary: Spruce up the file embeds a little more, hover state, icons, file size.

Test Plan:
Add a psd and pdf, see new icons. Check differential, still see icons there too. Test mobile, desktop.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16950