title: Django admin file upload with current model id
body: <p>I'm trying to create a simple photo gallery with the default Django admin. I'd like to save a sample photo for each gallery, but I don't want to keep the filname. Instead of the filename, I'd like to save the id of the model (<code>N.jpg</code>). But the first time I want to save the object the id does not exist. How could I know the next auto increment in the model, or somehow save the model data before the upload with <code>super.save</code> and after upload the file when <code>self.id</code> is exists? Is there a cool solution?</p>
  
  <p>Something like this:</p>
  
  <pre><code>def upload_path_handler(instance, filename):
      ext = filename extension
      return "site_media/images/gallery/{id}.{ext}".format(id=instance.nextincrement, ext=ext)
  
  class Gallery(models.Model):
      name  = models.TextField()
      image = models.FileField(upload_to=upload_path_handler)
  </code></pre>
  
  <p>And maybe store the filename in a different field.</p>