code: def test_no_format(self):
      buf = BytesIO(b"\x00" * 255)
  
      im = MockImageFile(buf)
      assert im.format is None
      assert im.get_format_mimetype() is None
  
imports: from io import BytesIO
  import pytest
  from PIL import BmpImagePlugin, EpsImagePlugin, Image, ImageFile, _binary, features
  from .helper import assert_image, assert_image_equal, assert_image_similar, fromstring, hopper, skip_unless_feature, tostring
label_desc: Use image node_id 6 for calling the TestPyDecoder obj's underlying member method code with example usage: obj.test_no_format() without return types