Fix GraphQL max upload size

[davimacedo]

This PR fixes one of the issues that is causing the bug of #5935

The max upload size was miscalculated and therefore only allowing the upload of small files.

[codecov]

Codecov Report

Merging #5940 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5940      +/-   ##
==========================================
- Coverage   93.65%   93.57%   -0.09%     
==========================================
  Files         156      156              
  Lines       10842    10842              
==========================================
- Hits        10154    10145       -9     
- Misses        688      697       +9

Impacted Files	Coverage Δ
src/GraphQL/ParseGraphQLServer.js	93.02% <100%> (ø)	⬆️
src/batch.js	89.79% <0%> (-2.05%)	⬇️
src/ParseServerRESTController.js	96.36% <0%> (-1.82%)	⬇️
src/Adapters/Storage/Mongo/MongoStorageAdapter.js	91.29% <0%> (-0.95%)	⬇️
src/Controllers/DatabaseController.js	94.62% <0%> (-0.18%)	⬇️
src/RestWrite.js	93.39% <0%> (-0.17%)	⬇️
...dapters/Storage/Postgres/PostgresStorageAdapter.js	96.6% <0%> (-0.09%)	⬇️

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