title: How include a filefield in a django admin model form that is not in the model
body: <p>I want to include a filefield in an admin model form that will be used to uploaded a file that will then be read and the contents will be used to update other fields in the same model. After processing the file itself is not needed, so i don't want the filefield in the model just in the form.</p>
  
  <p>I have no problem overriding save and processing the form myself, but I can't figure out how to include a filefield in my form that is not in the model.</p>