fix(files): adjust reactivity of files header outside of Vue instance

[susnux]

📝 Summary

• Resolves: #

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🚧 TODO

• ...

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

[mejo-]

Thanks @susnux! I incorporated similar changes into #2279