R1:afd86a042057

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…