rescue on attachment filename matching

URI.unescape and downcase are very unpredictable

fixes #CNVS-10762

Change-Id: I40c08257133498069139c9339692d32c4829f971
Reviewed-on: https://gerrit.instructure.com/29491
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>