[EmbedThumbnail] Support m4v Extention

[Neurognostic]

Support m4v Extention

Some platforms can serve m4v for mpeg video, this commit simply adds support for embedding thumbnails into m4v files.

Tested with both AtomicParsley and Mutagen. It is possible that m4v files could include DRM, but I don't have any DRM m4v files to test on. The existing error handling should probably be fine in those cases.

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• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

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What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)

Copilot Summary

🤖 Generated by Copilot at 1bbf619

Summary

🎞️🆕🛠️

Add thumbnail embedding for m4v files and improve error message. This allows users to embed thumbnails in more video formats and get clearer feedback on unsupported ones.

Sing, O Muse, of the skillful coder who devised
A way to adorn the m4v files with fair thumbnails
Like the cunning Hephaestus who forged the shield of Achilles
With many a splendid image of gods and men in battle.

Walkthrough

• Add support for embedding thumbnails in m4v files by modifying the file extension check (link)
• Update the error message for unsupported filetypes to include m4v (link)