code: def test_string_payload_with_multipart_content_type(self):
      msg = message_from_string(
          textwrap.dedent(
              """\
          Content-Type: multipart/mixed; charset="utf-8"
  
          sample text
          """
          ),
          policy=policy.default,
      )
      attachments = msg.iter_attachments()
      self.assertEqual(list(attachments), [])
  
imports: import unittest
  import textwrap
  from email import policy, message_from_string
  from email.message import EmailMessage, MIMEPart
  from test.test_email import TestEmailBase, parameterize
label_desc: Use image node_id 2 for calling the TestMIMEPart obj's underlying member method code with example usage: obj.test_string_payload_with_multipart_content_type() without return types