gistfile1.txt

<?php
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;
}
add_action( 'template_redirect', 'wpsites_attachment_redirect' );