R1:638bb7ae3f02

Allow files to be attached to Maniphest tasks while the task is being created.

Summary:
This commit lets users who are filing Maniphest tasks to attache files to them
right off the bat.

Test Plan:
{F3545}

and

{F3546}

Reviewers: epriestley, fmoo, aran

CC:

Differential Revision: 837