gh-80361: Fix TypeError in email.Message.get_payload() (GH-117994)

It was raised when the charset is rfc2231 encoded, e.g.:

   Content-Type: text/plain; charset*=ansi-x3.4-1968''utf-8

Lib/email/message.py

@@ -294,7 +294,7 @@ def get_payload(self, i=None, decode=False):
                 try:
                     bpayload = payload.encode('ascii', 'surrogateescape')
                     try:
-                        payload = bpayload.decode(self.get_param('charset', 'ascii'), 'replace')
+                        payload = bpayload.decode(self.get_content_charset('ascii'), 'replace')
                     except LookupError:
                         payload = bpayload.decode('ascii', 'replace')
                 except UnicodeEncodeError:

Lib/test/test_email/test_email.py

@@ -4181,6 +4181,21 @@ def test_8bit_in_uuencode_body(self):
         self.assertEqual(msg.get_payload(decode=True),
                          '<,.V<W1A; á \n'.encode('utf-8'))
 
+    def test_rfc2231_charset_8bit_CTE(self):
+        m = textwrap.dedent("""\
+        From: [email protected]
+        To: baz
+        Mime-Version: 1.0
+        Content-Type: text/plain; charset*=ansi-x3.4-1968''utf-8
+        Content-Transfer-Encoding: 8bit
+
+        pöstal
+        """).encode('utf-8')
+        msg = email.message_from_bytes(m)
+        self.assertEqual(msg.get_payload(), "pöstal\n")
+        self.assertEqual(msg.get_payload(decode=True),
+                         "pöstal\n".encode('utf-8'))
+
 
     headertest_headers = (
         ('From: [email protected]', ('From', '[email protected]')),

Misc/NEWS.d/next/Library/2024-04-17-18-00-30.gh-issue-80361.RstWg-.rst

@@ -0,0 +1,2 @@
+Fix TypeError in :func:`email.Message.get_payload` when the charset is :rfc:`2231`
+encoded.