Attachment page redirect with a custom field. Useful when you want to link your images in a [gallery] shortcode.

gistfile1.php

<?php

function media_custom_field_support() {
	add_post_type_support( 'attachment', 'custom-fields' );
}
add_action( 'init', 'media_custom_field_support' );

function do_attachment_redirect() {
	if( is_attachment() && $redirect = get_post_meta( $GLOBALS['post']->ID, 'redirect', true ) ) {
		wp_redirect( $redirect );
		exit;
	}
}
add_action( 'template_redirect', 'do_attachment_redirect' );