In the attempt to unify file uploading at workhorse level gitlab-org/gitlab-workhorse!230
we moved to a prefix-based tempfile creation in order to avoid upload collisions.

Artifacts and LFS uploads already set original_filename to workhorse provided filename

This commit add the same feature to `Gitlab::Middleware::Multipart`

I mistakenly concluded Rack::Multipart injects File instances into the
params. These should be UploadedFile instances. This reuses a mock
UploadedFile class we already had in GitLab.