[Form] fixes empty file-inputs get treated as extra field
merged branch Flask/ticket_8548 (PR #8575)

This PR was squashed before being merged into the 2.3 branch (closes #8575).

Discussion
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[Form] fixes empty file-inputs get treated as extra field

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8548, #8566
| License       | MIT

Commits
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e5fba3c [Form] fixes empty file-inputs get treated as extra field