BREAKING: Move form submit files to value property

[Mrtenz]

This removes the files property of the FormSubmitEvent in favour of the value property.

Breaking changes

• The type of FormSubmitEvent's value now includes File.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.36%. Comparing base (ea36386) to head (7dcb76a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2504   +/-   ##
=======================================
  Coverage   94.36%   94.36%           
=======================================
  Files         441      441           
  Lines        9068     9068           
  Branches     1396     1396           
=======================================
  Hits         8557     8557           
  Misses        511      511           

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