Hi Venkata,
Error Message- “file format or extension
is not valid”:
If the file format is different than the
extension then the MS Excel show the error “file format or extension is not
valid”. Can you let me know whether you are generating the xlsx file using
XlsIO with the Version property? Find the code snippet and online documentation
link to change the workbook version using XlsIO.
Code Snippet to change the Workbook Version:
//Select the version to be saved
workbook.Version = ExcelVersion.Excel2007; //or
//workbook.Version = ExcelVersion.Excel2010;
//Save it as "Excel2007" format.
workbook.SaveAs("Sample.xlsx");
Online Documentation
to change the Workbook Version:
http://help.syncfusion.com/ug/asp.net/xlsio/default.htm#!Documents/xlsx.htm
If still the issue is reproducing then share us the code
snippet which reproduces the issue.
Thanks,
Prasanth