R1:ce8a406424d4

Improve file preview in Maniphest

Summary:
Show large thumbnails of attached files in Maniphest.

Test Plan:
Looked at large thumbnails in Maniphest.

Reviewed By: jungejason
Reviewers: tomo, aran, jungejason, tuomaspelkonen
CC: anjali, aran, epriestley, jungejason
Differential Revision: 335