Redireccionar adjuntos al post asociado

redirect-attachments-to-parnt-post.php

add_action( 'template_redirect', 'wpsites_attachment_redirect' );
function wpsites_attachment_redirect(){
global $post;
if ( is_attachment() && isset($post->post_parent) && is_numeric($post->post_parent) && ($post->post_parent != 0) ) :
    wp_redirect( get_permalink( $post->post_parent ), 301 );
    exit();
    wp_reset_postdata();
    endif;
}

// fuente: http://wpsites.net/wordpress-tips/5-ways-to-redirect-attachment-pages-to-the-parent-post-url/