title: Xcode 7 warning: The launch image does not have a valid extension
body: <p>Xcode 7 has warnings for my launch image set. It reads:</p>
  
  <blockquote>
    <p>The launch image set "LaunchImage" references a file "Default@2x.PNG", but that file does not have a valid extension.</p>
  </blockquote>
  
  <p>I tried removing the image and put it back again with no luck. What is it warning about and how can I fix it?</p>