title: django form wizard doesn't keep the request.FILES when form is invalid?
body: <p>Lets say I have a form that has two fields - name and image, both required, and is part of a FormWizard.
  When I leave the "name" field blank but fill out the "image" field (which is an ImageField and so renders to a  tag) the form is not valid and so the wizard doesn't advance to the next step.
  but - the imagefield now "loses" its value - when I fill the name (which is the only error i see) and submitting the form again, the form is invalid because it says the image is missing.</p>
  
  <p>A similar thing happens if i fill both fields and then submit - the form validates, the wizard advances to the next form, and if I get back one step at the wizard the name is filled correctly as i left it but the ImageField loses its value again and I need to re-fill it.</p>
  
  <p>Is this normal behavior? is there something that can be done so this doesn't happen and the value is kept?</p>