chore(common): Backport FileTypeValidator fallback support to v10.4.17

[mag123c]

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PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior?

File validation fails when the buffer is too small or unidentifiable by magic number, even if the mimetype is correct.

Related issue: https://github.com/nestjs/nest/issues/14977

What is the new behavior?

• Adds a fallbackToMimetype option to FileTypeValidator
• If magic number detection fails, and fallback is enabled, the validator uses the mimetype
• Also improves buildErrorMessage() for clearer failure messages

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

This PR backports the fix originally merged in https://github.com/nestjs/nest/pull/14995 into the 10.4.17 line.

This was originally reported in #14977. All tests have been verified on the v10 codebase.