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

tests/test_mailgun_backend.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 
 from datetime import date, datetime
+from textwrap import dedent
+
 try:
     from email import message_from_bytes
 except ImportError:
@@ -171,9 +173,50 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
     def test_unicode_attachment_correctly_decoded(self):
         self.message.attach(u"Une pièce jointe.html", u'<p>\u2019</p>', mimetype='text/html')
         self.message.send()
-        files = self.get_api_call_files()
-        attachments = [value for (field, value) in files if field == 'attachment']
-        self.assertEqual(len(attachments), 1)
+
+        # Verify the RFC 7578 compliance workaround has kicked in:
+        data = self.get_api_call_data().decode("utf-8")
+        self.assertNotIn("filename*=", data)  # No RFC 2231 encoding
+        self.assertIn(u'Content-Disposition: form-data; name="attachment"; filename="Une pièce jointe.html"', data)
+
+        files = self.get_api_call_files(required=False)
+        self.assertFalse(files)  # files should have been moved to formdata body
+
+    def test_rfc_7578_compliance(self):
+        # Check some corner cases in the workaround that undoes RFC 2231 multipart/form-data encoding...
+        self.message.subject = u"Testing for filename*=utf-8''problems"
+        self.message.body = u"The attached message should have an attachment named 'vedhæftet fil.txt'"
+        # A forwarded message with its own attachment:
+        forwarded_message = dedent("""\
+            MIME-Version: 1.0
+            From: sender@example.com
+            Subject: This is a test message
+            Content-Type: multipart/mixed; boundary="boundary"
+
+            --boundary
+            Content-Type: text/plain
+
+            This message has an attached file with a non-ASCII filename.
+            --boundary
+            Content-Type: text/plain; name*=utf-8''vedh%C3%A6ftet%20fil.txt
+            Content-Disposition: attachment; filename*=utf-8''vedh%C3%A6ftet%20fil.txt
+
+            This is an attachment.
+            --boundary--
+            """)
+        self.message.attach(u"besked med vedhæftede filer", forwarded_message, "message/rfc822")
+        self.message.send()
+
+        data = self.get_api_call_data().decode("utf-8").replace("\r\n", "\n")
+        # Top-level attachment (in form-data) should have RFC 7578 filename (raw Unicode):
+        self.assertIn(
+            u'Content-Disposition: form-data; name="attachment"; filename="besked med vedhæftede filer"', data)
+        # Embedded message/rfc822 attachment should retain its RFC 2231 encoded filename:
+        self.assertIn("Content-Type: text/plain; name*=utf-8''vedh%C3%A6ftet%20fil.txt", data)
+        self.assertIn("Content-Disposition: attachment; filename*=utf-8''vedh%C3%A6ftet%20fil.txt", data)
+        # References to RFC 2231 in message text should remain intact:
+        self.assertIn("Testing for filename*=utf-8''problems", data)
+        self.assertIn(u"The attached message should have an attachment named 'vedhæftet fil.txt'", data)
 
     def test_embedded_images(self):
         image_filename = SAMPLE_IMAGE_FILENAME

tests/test_mailgun_integration.py

@@ -1,3 +1,7 @@
+# -*- coding: utf-8 -*-
+
+from __future__ import unicode_literals
+
 import os
 import logging
 import unittest
@@ -116,7 +120,7 @@ class MailgunBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
             track_opens=True,
         )
         message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
-        message.attach("attachment2.csv", "ID,Name\n1,3", "text/csv")
+        message.attach("vedhæftet fil.csv", "ID,Name\n1,3", "text/csv")
         cid = message.attach_inline_image_file(sample_image_path(), domain=MAILGUN_TEST_DOMAIN)
         message.attach_alternative(
             "<div>This is the <i>html</i> body <img src='cid:%s'></div>" % cid,
@@ -151,7 +155,7 @@ class MailgunBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
         self.assertEqual(len(attachments), 2)  # because inline image shouldn't be an attachment
         self.assertEqual(attachments[0]["filename"], "attachment1.txt")
         self.assertEqual(attachments[0]["content-type"], "text/plain")
-        self.assertEqual(attachments[1]["filename"], "attachment2.csv")
+        self.assertEqual(attachments[1]["filename"], "vedhæftet fil.csv")
         self.assertEqual(attachments[1]["content-type"], "text/csv")
 
         # No other fields are verifiable from the event data.