User Registration email notification in html format

Hi,
I know this kind of question has been asked before:

http://www.gravityhelp.com/forums/search.php?search=gform_notification_format&advanced=1&exact=1&posts=1&highlight=1&users=0&located=0&forums%5B%5D=0&maxcount=9999999&age=999999&direction=0&sort=date&order=0

My question is that does gform_notification_format have any affect on sign up user notification emails with the user registration add on?

I have the following code in my functions.php:

add_action("gform_notification_format", "gsale_set_notification_format", 10, 4);

function gsale_set_notification_format($format, $notification_type, $form, $lead){
         return "html"; // for testing - all emails as html
}

add_filter( 'wpmu_signup_user_notification_email', 'gsale_modify_signup_user_notification_message', 10, 4 );

function gsale_modify_signup_user_notification_message( $message, $user, $user_email, $key ) {

	$activation_link = trailingslashit( get_permalink( USER_ACTIVATION_PAGE ) ) .  "?page=gf_activation&key=$key";
	$path = STYLESHEET_URI;
	$message = file_get_contents( "{$path}template-user-registration-notification-email.html" );
	$message = str_replace( '{activation_link}', $activation_link, $message );
	return $message; // this is an html included file
}

Unfortunately it always appears as plain text in the email client. Suggestions or is this not implemented?