title: warning when opening generated xls file in excel 2007
body: <p>I get the following warning when opening an XML file with the ending <code>.xls</code> but I want to use it as <code>xls</code>:</p>
  
  <blockquote>
    <p>"The file you are trying to open, '[filename]', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"  (Yes | No | Help)</p>
  </blockquote>
  
  <p>Quoted from the MSDN blog article 'Excel 2007 Extension Warning On Opening Excel Workbook from a Web Site' <a href="https://web.archive.org/web/20160312023608/http://blogs.msdn.com/b/vsofficedeveloper/archive/2008/03/11/excel-2007-extension-warning.aspx" rel="nofollow noreferrer">archive link</a> <a href="http://blogs.msdn.com/b/vsofficedeveloper/archive/2008/03/11/excel-2007-extension-warning.aspx" rel="nofollow noreferrer">original link (broken)</a>.</p>
  
  <p>How to solve this?</p>
  
  <p>I use .xls with this source code:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet" xmlns:x="urn:schemas-microsoft-com:office:excel" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" xmlns:html="http://www.w3.org/TR/REC-html40"&gt;
  &lt;Worksheet ss:Name="Export"&gt;
  &lt;Table&gt;
  
  &lt;Row&gt; 
  &lt;Cell&gt;&lt;Data ss:Type="Number"&gt;3&lt;/Data&gt;&lt;/Cell&gt;
  
  &lt;Cell&gt;&lt;Data ss:Type="Number"&gt;22123497&lt;/Data&gt;&lt;/Cell&gt;
  
  &lt;/Row&gt;
  &lt;/Table&gt;
  &lt;/Worksheet&gt;
  &lt;/Workbook&gt;
  </code></pre>