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

[mag123c]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• 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?

• 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: #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
• No

Other information

This PR backports the fix originally merged in #14995 into the 10.4.17 line.

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