Merge: #21079: is_attachment now looks only at the value, ignoring parameters.

Lib/email/message.py

@@ -941,9 +941,7 @@ class MIMEPart(Message):
     @property
     def is_attachment(self):
         c_d = self.get('content-disposition')
-        if c_d is None:
-            return False
-        return c_d.lower() == 'attachment'
+        return False if c_d is None else c_d.content_disposition == 'attachment'
 
     def _find_body(self, part, preferencelist):
         if part.is_attachment:

Lib/test/test_email/test_message.py

@@ -729,7 +729,8 @@ class TestEmailMessageBase:
         self.assertTrue(m.is_attachment)
         m.replace_header('Content-Disposition', 'AtTachMent')
         self.assertTrue(m.is_attachment)
-
+        m.set_param('filename', 'abc.png', 'Content-Disposition')
+        self.assertTrue(m.is_attachment)
 
 
 class TestEmailMessage(TestEmailMessageBase, TestEmailBase):

Misc/NEWS

@@ -137,6 +137,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21079: Fix email.message.EmailMessage.is_attachment to return the
+  correct result when the header has parameters as well as a value.
+
 - Issue #22247: Add NNTPError to nntplib.__all__.
 
 - Issue #22366: urllib.request.urlopen will accept a context object