This PR was squashed before being merged into the 4.1-dev branch (closes #26502).

Discussion
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[Form] Add Bootstrap 4 style for field FileType

| Q             | A
| ------------- | ---
| Branch?       | master<!-- see below -->
| Bug fix?      | no
| New feature?  | yes <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #26464   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

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Commits
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df57718 [Form] Add Bootstrap 4 style for field FileType