fix content_type inferring with explicit filename

Change-Id: I5fa1cc6f000f5b22a819dcbb717286be0735d641
Reviewed-on: https://gerrit.instructure.com/42926
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Dave Jungst <dave@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>