from django.db.models import FileField as DjFileField

class FileField(DjFileField):

    # блокирую сохранение
    def pre_save(self, model_instance, add):
        return getattr(model_instance, self.attname)

    # генератор пути к файлу
    def generate_filename(self, instance, filename):
        return os.path.join(self.upload_to,
                            str(instance.id),
                            self.get_filename(filename))

class ItemImage(models.Model):
    file = ImageField(upload_to="uploads")

    def save(self, *args, **kwargs):
        super(ItemImage, self).save(*args, **kwargs)
        if self.file:
            # генерирую имя и повторно сохраняю
            self.file.name = self.file.field.generate_filename(self, self.file.name)
            self.file.save(self.file.name, self.file, save=False)
            ItemImage.objects.filter(id=self.id).update(file=self.file)