Describe the bug
When uploading a file via a signed POST request, it ends up with multipart/form-data as content type, instead of the content type defined in the file form field.

Excerpt from the HTTP request:

------geckoformboundary7eb9b63f990480415e846912c47f68a2
Content-Disposition: form-data; name="file"; filename="DSC_3619.JPG"
Content-Type: image/jpeg

To Reproduce
Upload a file via a signed POST request.

Expected behavior
File should have its original content type in storage.