fix: make opts.filename from #591 work with opts.keepExtensions

[tunnckoCore]

• Make introduced options.filename from feat: custom file (re)naming, sanitizing - use the new options.filename #591 work when options.keepExtensions: true.
• make keepExtensions work properly when multiple file extensions, like .tar.gz

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/cc @tunnckoCore @GrosSacASac

[codecov]

Codecov Report

Merging #597 into master will decrease coverage by 1.11%.
The diff coverage is 29.41%.

@@            Coverage Diff             @@
##           master     #597      +/-   ##
==========================================
- Coverage   83.93%   82.81%   -1.12%     
==========================================
  Files          14       14              
  Lines         635      646      +11     
  Branches      104      107       +3     
==========================================
+ Hits          533      535       +2     
- Misses         93      101       +8     
- Partials        9       10       +1     

Impacted Files	Coverage Δ
src/Formidable.js	73.23% <29.41%> (-3.00%)	⬇️

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