I need to create an excel template with extension .xltx.

The following code creates an xltx file. But when I open, I get an empty excel application with an error message "Excel cannot open the file 'NewTemplate.xltx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."

code:
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workbook = workbooks.Add(System.Reflection, Missing.Value);

workbook.SaveAs("D:\\NewTemplate.xltx", office2007.XlFileFormat.xlTemplate8, System.Reflection.Missing.Value, System.Reflection.Missing.Value, false, false, office2007.XlSaveAsAccessMode.xlNoChange, System.Reflection.Missing.Value, false, System.Reflection.Missing.Value, System.Reflection.Missing.Value, System.Reflection.Missing.Value);

workbooks.Close();
excelApplication.Quit();
/////////////////////////

What is wrong with this code?
Any help is very much appreciated.