title: Warning message when opening a generated excel file in office 07
body: <p>I generate an excel file using XML format outlined on stackoverflow in the following post:
  <a href="https://stackoverflow.com/questions/150339/generating-an-excel-file-in-asp-net">Generate excel file in .net</a></p>
  
  <p>However, when I open it up in excel 07, I get the following message:</p>
  
  <blockquote>
    <p>The file you are trying to open,
    "blah.xls", is in a different format
    than spefied by the file extension.
    Verify that the file i snot corrupted
    and is from a trusted source before
    opening the file. Do you want to open
    the file now?</p>
  </blockquote>
  
  <p>What is causing the error message and how can I get rid of it? Thanks!</p>