Use wp_check_filetype to determine file extension

[buutqn]

Fix WP_Customize_Media_Control::to_json() extension validation.

Trac ticket: #64557

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props buutqn, westonruter.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[Copilot]

Pull request overview

This PR fixes the extension validation logic in WP_Customize_Media_Control::to_json() by replacing a simple substring operation with WordPress's standard wp_check_filetype() function to properly determine file extensions.

Changes:

• Replaced substr($this->setting->default, -3) with wp_check_filetype($this->setting->default)['ext'] to correctly extract file extensions of any length and validate them against WordPress's known mime types

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[westonruter]

Note: In the future, it would be preferable to open a pull request to target trunk rather than the current release branch.

[westonruter]

Closing in favor of #10842