[5.3] System test in media manager to prevent a rename

[laoneo]

Summary of Changes

Pull request to actually check if an image can be renamed to a different one with a PHP file extension.

See https://developer.joomla.org/security-centre/961-20250301-core-malicious-file-uploads-via-media-managere-malicious-file-uploads-via-media-manager.html for more details.

[SniperSister]

I have tested this item ✅ successfully on c2f0fc4

Perfect, thank you!

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[richard67]

I have tested this item ✅ successfully on c2f0fc4

Verified in drone logs that the new system test was run, and reviewed that it tests the recent security fix.

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[richard67]

RTC

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