API: Fix file downloading

What does this MR do?

Fix presenting files in the API

Does this MR meet the acceptance criteria?

• Changelog entry added
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8587#note_22712373

/cc @xeizmendi