Allow validateFilename to return a string or Parse Error

[mpatnode]

This follows from parse-community/parse-server-s3-adapter#76

[codecov]

Codecov Report

Merging #6246 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6246      +/-   ##
==========================================
+ Coverage   93.98%   94.01%   +0.02%     
==========================================
  Files         169      169              
  Lines       11525    11529       +4     
==========================================
+ Hits        10832    10839       +7     
+ Misses        693      690       -3

Impacted Files	Coverage Δ
src/Controllers/FilesController.js	93.61% <100%> (+0.59%)	⬆️
src/RestWrite.js	93.75% <0%> (-0.17%)	⬇️
...dapters/Storage/Postgres/PostgresStorageAdapter.js	97.09% <0%> (ø)	⬆️
src/Controllers/SchemaController.js	96.63% <0%> (+0.21%)	⬆️
src/Adapters/Storage/Mongo/MongoTransform.js	88.97% <0%> (+0.29%)	⬆️
src/Routers/PushRouter.js	96.55% <0%> (+3.44%)	⬆️

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