Apply discount only for downloadable product

downloadable.php

function woo_discount_rules_exclude_cart_item_from_discount_method($skip, $cart_item){

	$product_ids_to_exclude = array();
	if(is_object($cart_item['data']) && method_exists($cart_item['data'], 'is_downloadable')) {
		if(!$cart_item['data']->is_downloadable()) {
		  $skip = true;
		}
	}
	return $skip;
}
add_filter('woo_discount_rules_exclude_cart_item_from_discount', 'woo_discount_rules_exclude_cart_item_from_discount_method', 10, 2);