Honour workhorse provided file name

What does this MR do?

Fixes a regression

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

In the attempt to unify file uploading at workhorse level gitlab-workhorse!230 (merged) 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 MR adds the same feature to Gitlab::Middleware::Multipart

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

gitlab-org/gitlab-ee#4878