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, there's no real reason to have it there.

Test Plan: Created a new task with attached files, verified they encoded as "[]"
instead of "true" in the database. Edited a task and didn't get a file control.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: danielraffel, aran

Differential Revision: 1003