title: "Format is not recognized" error while trying to sign xap-file with signtool
body: <p>I am trying to follow this tutotial:<br>
  <a href="http://channel9.msdn.com/learn/courses/Silverlight4/Overview/Overview/Moving-Beyond-the-Browser--Elevated-Trust-Applications/#_Toc256192962" rel="nofollow noreferrer">http://channel9.msdn.com/learn/courses/Silverlight4/Overview/Overview/Moving-Beyond-the-Browser--Elevated-Trust-Applications/#_Toc256192962</a><br>
  in order to sign my silverlight application.  </p>
  
  <p>But on the last step which is something like that:  </p>
  
  <p>signtool sign /v /f c:\Demo\TestOOBCodeSigningCA.pfx /p password c:\Demo\SilverlightApplication2\SilverlightApplication2.Web\ClientBin\SilverlightApplication2.xap</p>
  
  <p>I got the following error:  </p>
  
  <p>SignTool Error: This file format cannot be signed because it is not recognized<br>
  SignTool Error: An error occured while attempting to sign: c:\Demo\SilverlightApplication2\SilverlightApplication2.Web\ClientBin\SilverlightApplication2.xap     </p>
  
  <p>What can be the reason?</p>