Improve rules for embedding files received via email

Summary:
Ref T7199. Ref T7712. This improves the file rules for email:

• Embed visible images as thumbnails.
• Put all other file types in a nice list.

This "fixes" an issue caused by the opposite of the problem described in T7712 -- files being dropped if the default ruleset is too restrictive. T7712 is the real solution here, but use a half-measure for now.

Test Plan:

• Sent mail with two non-images and two images.
• Got a nice list of non-images and embeds of images.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7712, T7199

Differential Revision: https://secure.phabricator.com/D12235