extract multipart gem

Change-Id: Id4aa4d522bba2d87008d02249893a65387ec563c
Reviewed-on: https://gerrit.instructure.com/30052
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>