[stable32] fix(Viewer): Provide filename without root to the Viewer.

[backportbot]

Backport of #3239

Warning, This backport's changes differ from the original and might be incomplete ⚠️

Todo

• Review and resolve any conflicts
• Amend HEAD commit to remove the line stating to skip CI

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 2.22%. Comparing base (6eb7330) to head (64a5825).
⚠️ Report is 3 commits behind head on stable32.

Additional details and impacted files

@@           Coverage Diff            @@
##           stable32   #3241   +/-   ##
========================================
  Coverage      2.22%   2.22%           
========================================
  Files           107     107           
  Lines          6576    6576           
  Branches        121     121           
========================================
  Hits            146     146           
  Misses         6425    6425           
  Partials          5       5           

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[artonge]

/compile rebase /