don't double-escape filenames for attachments with root attachment

fixes #CNVS-13710

Change-Id: Icf3fd3b81ae841343047cea51663d1eeceed0e8d
Reviewed-on: https://gerrit.instructure.com/36778
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>