diff -r 119af59911a0 Lib/email/message.py
--- a/Lib/email/message.py	Mon Sep 05 11:43:18 2016 -0700
+++ b/Lib/email/message.py	Tue Sep 06 21:55:13 2016 -0400
@@ -1022,7 +1022,7 @@
         maintype, subtype = self.get_content_type().split('/')
         if maintype != 'multipart' or subtype == 'alternative':
             return
-        parts = self.get_payload()
+        parts = self.get_payload().copy()
         if maintype == 'multipart' and subtype == 'related':
             # For related, we treat everything but the root as an attachment.
             # The root may be indicated by 'start'; if there's no start or we
diff -r 119af59911a0 Lib/test/test_email/test_message.py
--- a/Lib/test/test_email/test_message.py	Mon Sep 05 11:43:18 2016 -0700
+++ b/Lib/test/test_email/test_message.py	Tue Sep 06 21:55:13 2016 -0400
@@ -732,6 +732,16 @@
         m.set_param('filename', 'abc.png', 'Content-Disposition')
         self.assertTrue(m.is_attachment())
 
+    def test_iter_attachments_mutation(self):
+        # We had a bug where iter_attachments was mutating the list.
+        m = self._make_message()
+        m.set_content('arbitrary text as main part')
+        m.add_related('more text as a related part')
+        m.add_related('yet more text as a second "attachment"')
+        orig = m.get_payload().copy()
+        self.assertEqual(len(list(m.iter_attachments())), 2)
+        self.assertEqual(m.get_payload(), orig)
+
 
 class TestEmailMessage(TestEmailMessageBase, TestEmailBase):
     message = EmailMessage