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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' );