Fix attachment mime unit tests
Fix attachment mime unit tests

The attachment upload form constructor changed in 2.5.x and a merge
from 2.0.x broke some unit tests. This patch corrects the constructor
usage in these tests and they now pass.

Testing Done:
Unit tests pass

Reviewed at https://reviews.reviewboard.org/r/7651/

sgallagher/reviewboard,reviewboard/reviewboard,brennie/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,davidt/reviewboard,brennie/reviewboard,brennie/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard,brennie/reviewboard,chipx86/reviewboard