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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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