feat(common): Add fallbackToMimetype support in FileTypeValidator

[mag123c]

Introduce fallbackToMimetype 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 enhanced buildErrorMessage() 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 using fallbackToMimetype: 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 to 10.4.17, as this issue was originally reported in v10.

• See discussion: File validation fails with MagicNumbersValidation #14977

Other information

[coveralls]

Pull Request Test Coverage Report for Build 837e18fc-8678-46c2-8d75-8d045ee82c96

Details

• 11 of 12 (91.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.003%) to 89.318%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/common/pipes/file/file-type.validator.ts	11	12	91.67%

Totals
Change from base Build 600c1ebc-e7f1-48f2-bb00-a8366dddbcf8:	-0.003%
Covered Lines:	7166
Relevant Lines:	8023

---

💛 - Coveralls

[Copilot]

Pull Request Overview

This PR introduces support for falling back to mimetype validation when magic number detection fails in FileTypeValidator, along with improved error messaging and additional unit tests.

• Added fallbackToMimetype option to FileTypeValidator
• Enhanced buildErrorMessage() to reflect fallback usage
• Included new unit tests for fallback behavior and error message variants

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/common/test/pipes/file/file-type.validator.spec.ts	Added tests for fallbackToMimetype logic and various error message scenarios
packages/common/pipes/file/file-type.validator.ts	Introduced fallbackToMimetype option in the validator and updated error messaging logic

[kamilmysliwiec]

LGTM

[mag123c]

Just submitted a backport PR for 10.4.17: #15003
Happy to adjust if needed!