//custom field gallery popup
/* For adding custom field to gallery popup */
function rt_image_attachment_fields_to_edit($form_fields, $post) {
    // $form_fields is a an array of fields to include in the attachment form
	// $post is nothing but attachment record in the database
	//     $post->post_type == 'attachment'
	// attachments are considered as posts in WordPress. So value of post_type in wp_posts table will be attachment
	// now add our custom field to the $form_fields array
	// input type="text" name/id="attachments[$attachment->ID][custom1]"
	$form_fields["rt-image-link"] = array(
		"label" => __("Slider Picture Link"),
		"input" => "text", // this is default if "input" is omitted
		"value" => get_post_meta($post->ID, "_rt-image-link", true),
                "helps" => __("Link zum Bild Schieberegler auf der Titelseite."),
	);
   return $form_fields;
}
// now attach our function to the hook
add_filter("attachment_fields_to_edit", "rt_image_attachment_fields_to_edit", null, 2);
function rt_image_attachment_fields_to_save($post, $attachment) {
	// $attachment part of the form $_POST ($_POST[attachments][postID])
        // $post['post_type'] == 'attachment'
	if( isset($attachment['rt-image-link']) ){
		// update_post_meta(postID, meta_key, meta_value);
		update_post_meta($post['ID'], '_rt-image-link', $attachment['rt-image-link']);
	}
	return $post;
}
// now attach our function to the hook.
add_filter("attachment_fields_to_save", "rt_image_attachment_fields_to_save", null , 2);