Fix content and content-type mismatch in files e2e test

What does this MR do and why?

Applies the same fix made in this spec: https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/5593.

Previously, we were sending a json body with a application/x-www-urlencoded content type, which caused an error after properly handling that content type.

Once merged, I will backport to 18.6, 18.5, and 18.4.

References

Related to https://gitlab.com/gitlab-org/quality/test-failure-issues/-/issues/19330

Screenshots or screen recordings

Before	After

How to set up and validate locally

bundle exec rspec qa/specs/features/api/3_create/repository/files_spec.rb succeeds

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