updating content type per http://filext.com/file-extension/XLSX. This is because FireFox will add .xls to the file if you choose to open it after downloading rather than saving.

Author: ssjman

src/main/groovy/pl/touk/excel/export/WebXlsxExporter.groovy

@@ -30,7 +30,7 @@ class WebXlsxExporter extends XlsxExporter {
 
     private WebXlsxExporter setHeaders(HttpServletResponse response, def filename) {
         response.setHeader("Content-disposition", "attachment; filename=$filename;")
-        response.setHeader("Content-Type", "application/vnd.ms-excel")
+        response.setHeader("Content-Type", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet")
         this
     }
 }