title: Sonata media validation at admin
body: <p>I'm trying to validate image. I saw answer at
   <a href="https://stackoverflow.com/q/15590505/1958355">Sonata Media: The file could not be found</a> </p>
  
  <p>How to validate image (width and height).
  I need help on that. There is no proper doc in net.</p>