Docs: note Mandrill broken attachment filenames

CHANGELOG.rst

@@ -25,6 +25,18 @@ Release history
 ^^^^^^^^^^^^^^^
     ..  This extra heading level keeps the ToC from becoming unmanageably long
 
+vNext
+-----
+
+*Unreleased changes in main branch*
+
+Other
+~~~~~
+
+* **Mandrill:** Document Mandrill's incorrect handling of non-ASCII attachment filenames.
+  (Thanks to `@Thorbenl`_ for reporting the issue and following up with MailChimp.)
+
+
 v8.5
 ----
 

docs/esps/mandrill.rst

@@ -135,11 +135,22 @@ as Mandrill's more complex list of name/content dicts.
 Limitations and quirks
 ----------------------
 
+**Non-ASCII attachment filenames will be garbled**
+  Mandrill's /messages/send API does not properly handle non-ASCII characters
+  in attachment filenames. As a result, some email clients will display
+  those characters incorrectly. The only workaround is to limit
+  attachment filenames to ASCII when sending through Mandrill.
+  (Verified and reported to MailChimp support 4/2022;
+  see `Anymail discussion #257`_ for more details.)
+
 **Envelope sender uses only domain**
   Anymail's :attr:`~anymail.message.AnymailMessage.envelope_sender` is used to
   populate Mandrill's `'return_path_domain'`---but only the domain portion.
   (Mandrill always generates its own encoded mailbox for the envelope sender.)
 
+.. _Anymail discussion #257:
+     https://github.com/anymail/django-anymail/discussions/257
+
 
 .. _mandrill-templates: