don't assign attachments attribute taht doesn't exist

refs CNVS-15830

Change-Id: I840ce48ca4293182fa21550586d6b0d08898c105
Reviewed-on: https://gerrit.instructure.com/43853
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>