Django admin file upload with current model id

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 (N.jpg). 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 super.save and after upload the file when self.id is exists? Is there a cool solution?

Something like this:

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)

And maybe store the filename in a different field.