Suggestion for #1295: use FILE_READ instead of new enum

[scpeters]

🦟 Bug fix

Proposed change to #1295

Summary

Implements the suggestion from https://github.com/gazebosim/sdformat/pull/1296/files#r1303216024

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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