Mailgun: Fix lost attachments with non-ASCII filenames.

Workaround requests/requests#4652 (urllib3/urllib3#303), where
uploaded files in multipart/form-data are improperly given RFC 2231
encoded filenames. That format is not accepted by Mailgun's API (and is
prohibited by RFC 7578), resulting in the attachments being silently
dropped.

Fix is to patch up the multipart/form-data before posting to remove
the RFC 2231 encoding.

Fixes #125

CHANGELOG.rst

@@ -38,6 +38,14 @@ Features
    backends (all but Amazon SES and SparkPost). Because this can expose API keys and
    other sensitive info in log files, it should not be used in production.
 
+Fixes
+~~~~~
+
+*  **Mailgun:** Fix problem where attachments with non-ASCII filenames would be lost.
+   (Works around Requests/urllib3 issue encoding multipart/form-data filenames in a way
+   that isn't RFC 7578 compliant. Thanks to `@decibyte`_ for catching the problem and
+   `@costela`_ for identifying the related Mailgun API oddity.)
+
 
 v4.2
 ----
@@ -833,6 +841,8 @@ Features
 .. _#115: https://github.com/anymail/issues/115
 
 .. _@calvin: https://github.com/calvin
+.. _@costela: https://github.com/costela
+.. _@decibyte: https://github.com/decibyte
 .. _@joshkersey: https://github.com/joshkersey
 .. _@Lekensteyn: https://github.com/Lekensteyn
 .. _@lewistaylor: https://github.com/lewistaylor