Fixed content-disposition in blob and files API endpoint

What does this MR do?

This MR fixes a regression from https://gitlab.com/gitlab-org/gitlab-ce/commit/9a5703ec82d394c2549f5afda7280c889d4cc373, where the content disposition in the blobs API endpoint returns attachment; filename=nil.

What are the relevant issue numbers?

Closes #55781 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer