fix(upload): implement iterable on input.files

[ph-fritsche]

What:

Implement iterable on target.files.

Why:

MDN states that HTMLInputElement.files is a FileList.
The browser also implements the iterable on this object, and some libraries rely on it:
Closes #576

Checklist:

• N/A Documentation
• Tests
• N/A Typings
• Ready to be merged

[codecov]

Codecov Report

Merging #603 (ff22621) into master (4f5b3ec) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #603   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           47        47           
  Lines          875       879    +4     
  Branches       330       330           
=========================================
+ Hits           875       879    +4     

Impacted Files	Coverage Δ
src/upload.ts	100.00% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 13.0.6 🎉

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