fallback on content_type in submissions api files instead
fallback on content_type in submissions api files instead

refs #LA-304

Change-Id: I339602fff5c880b597413b4bcc2a6417d44ba6fa
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/220411
QA-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Product-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms