def pre_save(self, model_instance, add):
        file = super(FileField, self).pre_save(model_instance, add)
        if file and not file._committed:
            # Commit the file to storage prior to saving the model
            file.save(file.name, file, save=False)
        return file
Returns field's value just before saving.
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.2/django/db/models/fields/files.py/FileField.pre_save