[stable31] feat: add downloadCallback for Viewer handler

[backportbot]

Backport of PR #7889

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.19%. Comparing base (648c479) to head (de4eef4).
⚠️ Report is 3 commits behind head on stable31.

Files with missing lines	Patch %	Lines
src/viewer.js	0.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           stable31    #7927      +/-   ##
============================================
- Coverage     51.20%   51.19%   -0.02%     
============================================
  Files           292      292              
  Lines         41825    41834       +9     
  Branches        869      869              
============================================
  Hits          21415    21415              
- Misses        20290    20299       +9     
  Partials        120      120              

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