[#2699] Uploads in repeating group

[SilviaAmAm]

Fixes #2699

[codecov]

Codecov Report

Base: 93.99% // Head: 94.00% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (f00c1f8) compared to base (cf66825).
Patch coverage: 97.29% of modified lines in pull request are covered.

❗ Current head f00c1f8 differs from pull request most recent head b8b10bd. Consider uploading reports for the commit b8b10bd to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2704      +/-   ##
==========================================
+ Coverage   93.99%   94.00%   +0.01%     
==========================================
  Files         595      595              
  Lines       17997    18028      +31     
  Branches     2034     2045      +11     
==========================================
+ Hits        16916    16947      +31     
- Misses        786      787       +1     
+ Partials      295      294       -1     

Impacted Files	Coverage Δ
src/openforms/submissions/attachments.py	89.07% <97.05%> (-3.39%)	⬇️
src/openforms/formio/utils.py	100.00% <100.00%> (ø)
...forms/registrations/contrib/camunda/serializers.py	100.00% <0.00%> (ø)
src/openforms/conf/base.py	93.02% <0.00%> (+0.03%)	⬆️
src/openforms/utils/api/drf_jsonschema.py	90.32% <0.00%> (+29.60%)	⬆️

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[sergei-maertens]

please create a ticket to refactor this upload handling into a more robust, easier testable setup. This suffices as a bandaid, but it won't hold up in the future.

[SilviaAmAm]

New issue: #2713

[sergei-maertens]

Backported in 667358a