spec: fix fragile file specs

the filename may not match the display_name if the newly uploaded
attachment is not a root attachment

refs CNVS-14376

Change-Id: I9e108e9311157fd6b91230090f1b114109b67a7f
Reviewed-on: https://gerrit.instructure.com/38017
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>