spec: fix assignments attachment spec

fixes CNVS-10518

Change-Id: I4caa8b8a9c5f977973cca6f84f9e0d8b9452c649
Reviewed-on: https://gerrit.instructure.com/28599
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>