fix Content-Type in kaltura multipart request

test plan:
 - import content including a video file into a course
   (such as the QTI file attached to CNVS-11602)
 - the import should complete

fixes CNVS-11602

Change-Id: I0e607dd0714df5456ec7d5da47347e4ab89c7597
Reviewed-on: https://gerrit.instructure.com/31408
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>