IntroducefallbackToMimetype option to allow fallback to mimetype validation when magic number detection fails (e.g., for small or undetectable buffers like text or CSV files).

Also enhancedbuildErrorMessage() to reflect precise validation failure reasons depending on buffer presence and mimetype state.

Added unit tests to cover fallback logic and error message variants.

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?

Currently,FileTypeValidator only relies on magic number detection.
When the file buffer is too short or lacks a detectable signature, validation fails even if the mimetype is correctly set.

Issue Number:#14977

• Allows developers to enable fallback to mimetype validation usingfallbackToMimetype: true
• Better error messaging when fallback is triggered or buffer is missing
• Adds relevant unit tests for fallback behavior and error formatting

What is the new behavior?

Does this PR introduce a breaking change?

• Yes

• No

• A separate PR will be submitted to backport this feature to10.4.17, as this issue was originally reported in v10.

• See discussion:File validation fails with MagicNumbersValidation #14977

Other information