gh-123742: Fixed get_payload not being able to parse headers with spaces

Lib/email/message.py

@@ -287,7 +287,7 @@ def get_payload(self, i=None, decode=False):
             raise TypeError('Expected list, got %s' % type(self._payload))
         payload = self._payload
         # cte might be a Header, so for now stringify it.
-        cte = str(self.get('content-transfer-encoding', '')).lower()
+        cte = str(self.get('content-transfer-encoding', '')).lower().strip(' ')
         # payload may be bytes here.
         if not decode:
             if isinstance(payload, str) and utils._has_surrogates(payload):

Misc/NEWS.d/next/Library/2024-09-06-11-58-17.gh-issue-123742.jSBNUv.rst

@@ -0,0 +1,3 @@
+Fixed :meth:`!email.Message.get_payload` failing to parse the
+``content-transfer-encoding`` field with extra spaces. It will now
+automatically strip spaces from ``content-transfer-encoding``.