diff --git a/Lib/email/message.py b/Lib/email/message.py
index aa46deb..124071d 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -941,9 +941,7 @@
     @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:
diff --git a/Lib/test/test_email/test_message.py b/Lib/test/test_email/test_message.py
index c761c62..51ddf36 100644
--- a/Lib/test/test_email/test_message.py
+++ b/Lib/test/test_email/test_message.py
@@ -729,7 +729,8 @@
         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):
diff --git a/Misc/NEWS b/Misc/NEWS
index 1ba6a4a..c5afa50 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -32,6 +32,9 @@
 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 #4180: The warnings registries are now reset when the filters