title: File upload after model save on Django admin
body: <p>I am using a file upload in my Django model like this : </p>
  
  <pre><code> def upload_path(self, filename):
      return 'upload/actualities/%s/%s' % (self.id, filename)
  
  photo = models.ImageField(upload_to=upload_path)
  </code></pre>
  
  <p>and my adminModel is : </p>
  
  <pre><code>from actualities.models import *
  from django.contrib import admin  
  
  class ActualityAdmin(admin.ModelAdmin):
       class Media:
           js = ('/static/js/tiny_mce/tiny_mce.js', '/static/js/textareas.js')
  
   admin.site.register(Actuality, ActualityAdmin)
  </code></pre>
  
  <p>Everything works fine except when i edit mu model because it has an id. But when I create it, the file upload happens before the model saving... So i put my file in /media/actualities/None/filename.jpg, and I want /media/2/filename.jpg</p>
  
  <p>How can I force to make the file upload after the model saving?</p>
  
  <p>Thank you!!!</p>