make add files work with S3

closes CNVS-14823

test plan:
  - add <AllowedHeader>content-type</AllowedHeader>S3 CORs config
  - enable S3 uploads in config/file_store.yml and config/amazon_s3.yml
  - add a file using the ui
    - file should upload successfully

Change-Id: I9fb3061def386a4827bee04c7727b336f9a311ba
Reviewed-on: https://gerrit.instructure.com/39935
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>
QA-Review: Jason Madsen <jmadsen@instructure.com>