Mailgun/SendGrid inbound: workaround Django filename issue

Workaround for Django multipart/form-data limitation
where certain attachment filenames cause fields to be dropped
or to end up in request.POST rather than request.FILES.

Handle the MultiValueDictKeyError in inbound webhooks when
this has occurred. Also update docs to recommend avoiding
the problem by using Mailgun and SendGrid's "raw MIME" options.

Also handle reported cases of empty, duplicate keys in Mailgun's
content-id-map.

Fixes #272

tests/test_mailgun_inbound.py

@@ -15,7 +15,7 @@ from anymail.webhooks.mailgun import MailgunInboundWebhookView
 from .test_mailgun_webhooks import (
     TEST_WEBHOOK_SIGNING_KEY, mailgun_sign_payload,
     mailgun_sign_legacy_payload, querydict_to_postdict)
-from .utils import sample_image_content, sample_email_content
+from .utils import sample_image_content, sample_email_content, encode_multipart, make_fileobj
 from .webhook_cases import WebhookTestCase
 
 
@@ -104,6 +104,7 @@ class MailgunInboundTestCase(WebhookTestCase):
         att2.name = 'image.png'
         email_content = sample_email_content()
         att3 = BytesIO(email_content)
+        att3.name = '\\share\\mail\\forwarded.msg'
         att3.content_type = 'message/rfc822; charset="us-ascii"'
         raw_event = mailgun_sign_legacy_payload({
             'message-headers': '[]',
@@ -125,6 +126,7 @@ class MailgunInboundTestCase(WebhookTestCase):
         self.assertEqual(attachments[0].get_filename(), 'test.txt')
         self.assertEqual(attachments[0].get_content_type(), 'text/plain')
         self.assertEqual(attachments[0].get_content_text(), 'test attachment')
+        self.assertEqual(attachments[1].get_filename(), 'forwarded.msg')  # Django strips paths
         self.assertEqual(attachments[1].get_content_type(), 'message/rfc822')
         self.assertEqualIgnoringHeaderFolding(attachments[1].get_content_bytes(), email_content)
 
@@ -135,6 +137,68 @@ class MailgunInboundTestCase(WebhookTestCase):
         self.assertEqual(inline.get_content_type(), 'image/png')
         self.assertEqual(inline.get_content_bytes(), image_content)
 
+    def test_filtered_attachment_filenames(self):
+        # Make sure the inbound webhook can deal with missing fields caused by
+        # Django's multipart/form-data filename filtering. (The attachments are lost,
+        # but shouldn't cause errors in the inbound webhook.)
+        filenames = [
+            "", "path\\", "path/"
+            ".", "path\\.", "path/.",
+            "..", "path\\..", "path/..",
+        ]
+        num_attachments = len(filenames)
+        payload = {
+            "attachment-%d" % (i+1): make_fileobj("content", filename=filenames[i], content_type="text/pdf")
+            for i in range(num_attachments)
+        }
+        payload.update({
+            'message-headers': '[]',
+            'attachment-count': str(num_attachments),
+        })
+
+        # Must do our own multipart/form-data encoding for empty filenames:
+        response = self.client.post('/anymail/mailgun/inbound/',
+                                    data=encode_multipart("BoUnDaRy", mailgun_sign_legacy_payload(payload)),
+                                    content_type="multipart/form-data; boundary=BoUnDaRy")
+        self.assertEqual(response.status_code, 200)
+        kwargs = self.assert_handler_called_once_with(self.inbound_handler, sender=MailgunInboundWebhookView,
+                                                      event=ANY, esp_name='Mailgun')
+
+        # Different Django releases strip different filename patterns.
+        # Just verify that at least some attachments got dropped (so the test is valid)
+        # without causing an error in the inbound webhook:
+        attachments = kwargs['event'].message.attachments
+        self.assertLess(len(attachments), num_attachments)
+
+    def test_unusual_content_id_map(self):
+        # Under unknown conditions, Mailgun appears to generate a content-id-map with multiple
+        # empty keys (and possibly other duplicate keys). We still want to correctly identify
+        # inline attachments from it.
+        raw_event = mailgun_sign_legacy_payload({
+            'message-headers': '[]',
+            'attachment-count': '4',
+            'content-id-map': '{"": "attachment-1", "": "attachment-2",'
+                              ' "<abc>": "attachment-3", "<abc>": "attachment-4"}',
+            'attachment-1': make_fileobj("att1"),
+            'attachment-2': make_fileobj("att2"),
+            'attachment-3': make_fileobj("att3"),
+            'attachment-4': make_fileobj("att4"),
+        })
+
+        response = self.client.post('/anymail/mailgun/inbound/', data=raw_event)
+        self.assertEqual(response.status_code, 200)
+        kwargs = self.assert_handler_called_once_with(self.inbound_handler, sender=MailgunInboundWebhookView,
+                                                      event=ANY, esp_name='Mailgun')
+        event = kwargs['event']
+        message = event.message
+        self.assertEqual(len(message.attachments), 0)  # all inlines
+        inlines = [part for part in message.walk() if part.is_inline_attachment()]
+        self.assertEqual(len(inlines), 4)
+        self.assertEqual(inlines[0]["Content-ID"], "")
+        self.assertEqual(inlines[1]["Content-ID"], "")
+        self.assertEqual(inlines[2]["Content-ID"], "<abc>")
+        self.assertEqual(inlines[3]["Content-ID"], "<abc>")
+
     def test_inbound_mime(self):
         # Mailgun provides the full, raw MIME message if the webhook url ends in 'mime'
         raw_event = mailgun_sign_legacy_payload({