Удаление всех миниатюр у отдельного типа записи

functions.php

function delete_all_attachments_by_post_type() {

	global $wpdb;

	$attachments = $wpdb->get_results(
		"SELECT meta_value as att_id
                FROM $wpdb->postmeta
		             INNER JOIN $wpdb->posts
		                        ON ($wpdb->posts.ID = $wpdb->postmeta.post_id AND $wpdb->posts.post_type = 'courses')
                WHERE meta_key = '_thumbnail_id'"
	);

	foreach ( $attachments as $attachment ) {
		wp_delete_attachment( $attachment->att_id, true );
	}
}