The problematic code:
$extensions = explode( ' ', $magic->getExtensionsForType( $mimeType ) );
if ( count( $extensions ) ) {
$extension = $extensions[0];
}
If $mimeType is not known to MediaWiki,
$magic->getExtensionsForType($mimeType) will return null.
explode( ' ', null ) is [""]. (Thank you so much, PHP!)
This means $extensions is nonempty (it contains the empty string), so
$extension is set to ''.
Change-Id: Icf387a9c93cb7351c2f48c69f413e7ad2224ba6b
0a82600a27