retain attachment content_type on cross shard user merge

test plan:
* create a user
* create another user on another shard
* upload a previewable file (such as an image) for a user
* merge the user with the file into the other user
* after jobs have run, the file should show up in the
 final user's folder, and should still be previewable

fixes #CNVS-13059

Change-Id: I20eec5d8b9dac06a72dd3f7a7018cb5034426080
Reviewed-on: https://gerrit.instructure.com/35230
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>