title: Excel file Created in C# cannot be opened
body: <p>After Creating an Excel file using C#, I'm trying to open the file, and get the following exception: </p>
  
  <p><strong>"excel cannot open the file because the file format or file extension is not valid. verify that the file has not been corrupted and that the file extention matches the format of the file."</strong></p>
  
  <p>Any suggestions?</p>