如何修复文件格式和扩展名不匹配？

I created a code in c# which creates and saves excel file. The code can successfully create and save excel file, but when I open the excel file created, it displays a warning message telling "The file format and extension of 'filename.xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?". I am using the following code:I created a code in c# which creates and saves