Fix function signature, use default random boundary

[S7evinK]

Fixes the function signature of parseMultipartResponse and uses the default random boundary when creating a new multipart response.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 68.28%. Comparing base (002fed3) to head (ecf5a95).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
mediaapi/routing/download.go	33.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3422      +/-   ##
==========================================
- Coverage   68.30%   68.28%   -0.02%     
==========================================
  Files         513      513              
  Lines       47029    47026       -3     
==========================================
- Hits        32123    32114       -9     
- Misses      10909    10913       +4     
- Partials     3997     3999       +2     

Flag	Coverage Δ
unittests	53.29% <33.33%> (-0.01%)	⬇️

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