function myprefix_redirect_attachment_page() {
    if ( is_attachment() ) {
        global $post;
        if ( $post && $post->post_parent ) {
            wp_redirect( esc_url( get_permalink( $post->post_parent ) ), 301 );
            exit;
        } else {
            wp_redirect( esc_url( home_url( '/' ) ), 301 );
            exit;
        }
    }
}
add_action( 'template_redirect', 'myprefix_redirect_attachment_page' );