Thank you for your reply.
I guess I have not explained the situation clearly with my last post. The ExportToExcelML exports the data from RadGridView in Excel XML format. Files which contain data in this file format should have .xml extension. On the other hand we consider .xml might be confusing for end-users as they do not associate xml with Excel. We use xls file extension just because users recognize it and associate it with Excel and because Excel can open such a file. By such a file I mean a file that has a "wrong" file extension that does not correspond to the internal file format. This does not apply to the xslx file extension, Excel cannot open a file with Excel XML file format and xlsx file extension. As a result if we are to support xlsx extension, we will have to implement an exporter that would export data in xlsx file format and this currently we do not have a time frame specified about that. Of course, if there are many users who want this feature, we will increase its priority in our development plans.
I hope this makes things clearer. Do not hesitate to write back with any further questions.
the Telerik team