Filter by post type.

filter-post-type.php

<?php
/**
 * Add post type filter support for search.
 *
 * @param array $query
 *
 * @since 1.0.0
 */

public function filter_post_type( $query ) {
	if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
		$filter_post_type = ! empty( $_GET[ 'filter_post_type' ] ) ? $_GET[ 'filter_post_type' ] : '';

		if ( ! empty( $filter_post_type ) &&  post_type_exists( $filter_post_type ) ) {
			$query->set( 'post_type', $filter_post_type );
		}
	}

	return $query;
}

add_filter( 'pre_get_posts', 'filter_post_type' );