Add validator for multipart forms

[boba-keyost]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 40.12%. Comparing base (b19848c) to head (6e8fab0). Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
- Coverage   40.72%   40.12%   -0.61%     
==========================================
  Files          16       16              
  Lines        6607    10385    +3778     
==========================================
+ Hits         2691     4167    +1476     
- Misses       3440     5708    +2268     
- Partials      476      510      +34     

Flag	Coverage Δ
unittests	40.12% <100.00%> (-0.61%)	:arrow_down:

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