'FileField.save_form_data(), if passed a truthy value, updates its
instance attribute.'

def test_changed(self):

    d = Document(myfile='something.txt')
    self.assertEqual(d.myfile, 'something.txt')
    field = d._meta.get_field('myfile')
    field.save_form_data(d, 'else.txt')
    self.assertEqual(d.myfile, 'else.txt')
