fix attachment_fu callbacks for rails 3

Change-Id: I7edc978b22fee7e4cb4c8c9c97188858c64b3795
Reviewed-on: https://gerrit.instructure.com/27677
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>