Interop excel

I went through hell getting the right saveas format
until I started using
XlFileFormat.xlXMLSpreadsheet

thus:

workBook.SaveAs(path,

                 XlFileFormat.xlXMLSpreadsheet,

                Type.Missing, Type.Missing,

                false, false, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange,

                Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);