R1:97f38b4e2733

Fix some minor issues with Maniphest file/attachment handling

Summary:
@danielraffel is reporting an issue with file attachments which I can't
reproduce, but I did find a couple of minor things.

- Elsewhere, we store array() as the value of these PHID dictionaries (the
idea being that we might store metadata there some day). While we may or may not
do this, we should at least be consistent.
- When you edit a task, there's a file upload control but it doesn't actually
do anything. Just don't show it…