function attachment_search( $query ) {
    if ( $query->is_search ) {
       $query->set( 'post_type', array( 'post', 'attachment' ) );
       $query->set( 'post_status', array( 'publish', 'inherit' ) );
    }
 
   return $query;
}

add_filter( 'pre_get_posts', 'attachment_search' );