xls download issue

switch download xls to xlsx to avoid random warning message.
https://api.highcharts.com/highcharts/exporting.csv?_ga=2.24980154.1646661079.1666896720-1791866074.1665168726 
https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/export-data/xlsx/