using Syncfusion.XlsIO;
ExcelEngine excelEngine = new ExcelEngine();
IApplication application = excelEngine.Excel;
IWorkbook workbook = excelEngine.Excel.Workbooks.Open(toBeSavefilePath);
IWorksheet sheet1 = workbook.Worksheets[0];
Ref url: http://www.syncfusion.com/forums/73976/cant-open-any-xlsx-files