title: Convert xlsx to xls issue
body: <p>I am trying to convert a <code>.xlsx</code> file to <code>.xls</code> and it seems to be working fine, however when I open the <code>.xls</code> file I get a warning message "</p>
  
  <blockquote>
    <p>The file you are trying to open 'filename', is in a different format
    than specified by the file extension.</p>
    
    <p>Verify that the file is not corrupted and >from a trusted source before opening the >file.  Do you want to open the file now?".... </p>
  </blockquote>
  
  <p>Everything looks fine when I open it, but I do not understand why this is happening.  My next step in this program is to import the data from the xls into SQL, but I am afraid this will cause issues for that.</p>
  
  <p>Here is the line of code where I call the SaveAs method to change the file extension of the file.  </p>
  
  <pre><code>wb.SaveAs("filename.xls", FileFormat: Microsoft.Office.Interop.Excel.XlFileFormat.xlOpenXMLWorkbook);
  </code></pre>
  
  <p>I initially just had </p>
  
  <pre><code>wb.SaveAs("filename.xls"); 
  </code></pre>
  
  <p>and after I got the error I did some more digging around and found the <code>xlOpenXMLWorkbook</code>, but does not seem to help.</p>
  
  <p>Any info that could help me understand why this is happening would be much appreciated.</p>