File not save correclty with save as option in c#

Steps:
1. I uploaded the file demo.xlsx
2. on button click event
 protected void btnUpload_Click(object sender, EventArgs e)
  {
     string file = FileUpload1.PostedFile.FileName;      FileUpload1.SaveAs(file.Replace("xlsx", "html"));   }
3. I have to save the excel file as html
4. But it not save correctly
Is any solution for it