Source

File: wp-includes/functions.php

function wp_ext2type( $ext ) {
	$ext = strtolower( $ext );

	$ext2type = wp_get_ext_types();
	foreach ( $ext2type as $type => $exts ) {
		if ( in_array( $ext, $exts, true ) ) {
			return $type;
		}
	}
}