Improve UI hints and error messages for supported file types

Summary:
We give you a pretty bad error right now if your server doesn't have, say, png support, saying "only png is supportd loololloo".

Instead, show you which formats are supported in the error messsage, and tell you upfront.

Test Plan: Tried to upload supported and unsupported images, got appropriate errors and supported format text.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T981

Differential Revision: https://secure.phabricator.com/D1894