[stable32] feat: add downloadCallback for Viewer handler

[backportbot]

Backport of PR #7889

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.12%. Comparing base (78fa2c2) to head (d8aae12).
⚠️ Report is 3 commits behind head on stable32.

Files with missing lines	Patch %	Lines
src/viewer.js	0.00%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           stable32    #7926      +/-   ##
============================================
- Coverage     52.14%   52.12%   -0.02%     
============================================
  Files           499      499              
  Lines         43088    43103      +15     
  Branches       1063     1063              
============================================
  Hits          22468    22468              
- Misses        20511    20526      +15     
  Partials        109      109              

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