Throw an exception on file absence in multipart

[eplatonoff]

Throws an error in case of a malformed request with incorrect or absent file fields for correspondent keys in the map.

[codecov[bot]]

Codecov Report

Merging #21 (6a6c3f1) into master (0d6eb19) will decrease coverage by 0.43%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   99.55%   99.11%   -0.44%     
==========================================
  Files           1        1              
  Lines         224      227       +3     
==========================================
+ Hits          223      225       +2     
- Misses          1        2       +1     

Flag	Coverage Δ
unittests	99.11% <75.00%> (-0.44%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
starlette_graphene3.py	99.11% <75.00%> (-0.44%)	:arrow_down:

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[ciscorn]

Thank you!