Changing WP Media Gallery Fields

function custom_media_gallery_fields($form_fields, $post) {
    $form_fields["custom_link"] = array(
        "label" => __("External URL"),
        "input" => "text",
        "value" => get_post_meta($post->ID, "_custom_link", true), // save as meta
        "helps" => __("Your external link.")
    );

    unset( $form_fields['url'] ); // [optional] remove original URL field

    return $form_fields;
}
add_filter("attachment_fields_to_edit", "custom_media_gallery_fields", null, 2);