WIP: gitlab-org/gitlab-ce#34963 - specify content-type for '/projects/:id/repository/files/:file_path'

What does this MR do?

specify content-type for '/projects/:id/repository/files/:file_path'

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

Why was this MR needed?

When the file name was added to the file_path, it was triggering a return of type text/plain because of the setting in api.rb - content_type :txt, "text/plain", removing that line caused some random (to me) tests to fail, so I went with this tactic instead

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

What are the relevant issue numbers?

Closes #34963 (moved)