Fixed #18967 -- Don't base64-encode message/rfc822 attachments.

Thanks Michael Farrell for the report and his work on the fix.

docs/topics/email.txt

@@ -319,6 +319,18 @@ The class has the following methods:
 
        message.attach('design.png', img_data, 'image/png')
 
+    .. versionchanged:: 1.7
+
+      If you specify a ``mimetype`` of ``message/rfc822``, it will also accept
+      :class:`django.core.mail.EmailMessage` and :py:class:`email.message.Message`.
+
+      In addition, ``message/rfc822`` attachments will no longer be
+      base64-encoded in violation of :rfc:`2046#section-5.2.1`, which can cause
+      issues with displaying the attachments in `Evolution`__ and `Thunderbird`__.
+
+      __ https://bugzilla.gnome.org/show_bug.cgi?id=651197
+      __ https://bugzilla.mozilla.org/show_bug.cgi?id=333880
+
 * ``attach_file()`` creates a new attachment using a file from your
   filesystem. Call it with the path of the file to attach and, optionally,
   the MIME type to use for the attachment. If the MIME type is omitted, it
@@ -326,8 +338,6 @@ The class has the following methods:
 
     message.attach_file('/images/weather_map.png')
 
-.. _DEFAULT_FROM_EMAIL: ../settings/#default-from-email
-
 Sending alternative content types
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~