Manual:$wgTrivialMimeDetection

MIME types: $wgTrivialMimeDetection
Switch for trivial MIME detection.
Introduced in version:1.5.0
Removed in version:still in use
Allowed values:(boolean)
Default value:false

Details edit

This setting is for internal use within MediaWiki only and is not normally required by end users.

This is set to true by thumb.php to disable all fancy MIME detection routines, because only a few types of images are needed and file extensions can be trusted.