download formatted files from google doc when available

google docs allows you to specify the format in additional
parameters in the download url, so we can download .doc or
and .xlsx files instead of always plain 'text/html' content

test plan:
* enable the google docs and crocodoc plugins
* create a course with a file upload assignment
* as a student in the course, enable google docs
 integration in user settings
* submit a file to the assignment through the google
 docs tab
* the submitted file should open up in crocodoc

fixes #CNVS-1503

Change-Id: I00ad9ac25a0dd6c46e5b56d2e566ab88b6550054
Reviewed-on: https://gerrit.instructure.com/33994
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>