I have this code to generate an Excel file and download it:
ExcelFile myExcelFile = new ExcelFile();
ExcelWorksheet ws = myExcelFile.Worksheets.Add("Page 1");
myExcelFile.Save(Response, "asd.xlsx"); //this is for download
Either file type or file extension is not valid so Excel cannot open
this file. Confirm that file is not broken or file extension matches
Try to save the fire as an xls-file instead of an xlsx-file. I'm guessing that the default is xls. Excel can get confused if the file extension is wrong.