Currently, the RadGrid exports files to Excel using the legacy .xls format/extension. However, all Office documents starting with Office 2007 and later support the .xlsx format. Therefore, the RadGrid should support the ability to export data to either the legacy .xls format or the newer .xlsx format through attribute settings in the RadGrid control.
The XLSX export is already supported by RadGrid. You can test it at http://demos.telerik.com/aspnet-ajax/grid/examples/functionality/exporting/excel-export/defaultcs.aspx
I too would like to see this added, it's now 6 years after these format's where introduced and yet we do not have support for them. In the financial world it is important to support these format's especially with MS now flagging XLS files containing none standard content (ie. html) as issues. These exports should be across all relevant controls ie. RadGrid in the WPF world suffers this issue as well.