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/utils.py

@@ -5,12 +5,12 @@ import uuid
 import warnings
 from base64 import b64decode
 from contextlib import contextmanager
-from io import StringIO
+from io import BytesIO, StringIO
 from pathlib import Path
 from unittest import TestCase
 from unittest.util import safe_repr
 
-from django.test import Client
+import django.test.client
 
 
 def decode_att(att):
@@ -177,7 +177,7 @@ class AnymailTestMixin(TestCase):
             sys.stdout = old_stdout
 
 
-class ClientWithCsrfChecks(Client):
+class ClientWithCsrfChecks(django.test.Client):
     """Django test Client that enforces CSRF checks
 
     https://docs.djangoproject.com/en/stable/ref/csrf/#testing
@@ -227,3 +227,35 @@ def dedent_bytes(text):
     if margin:
         text = re.sub(b'(?m)^' + margin, b'', text)
     return text
+
+
+def make_fileobj(content, filename=None, content_type=None, encoding=None):
+    """
+    Returns a file-like object that can be used in Django test Client
+    post data to simulate an uploaded file.
+    """
+    # The logic that unpacks this is in django.test.client.encode_file.
+    if isinstance(content, str):
+        content = content.encode(encoding or 'utf-8')
+    fileobj = BytesIO(content)
+    if filename is not None:
+        fileobj.name = filename
+    if content_type is not None:
+        fileobj.content_type = content_type
+    return fileobj
+
+
+def encode_multipart(boundary, data):
+    """
+    Version of :func:`django.test.client.encode_multipart` that allows
+    empty filenames. (The original function substitutes the field's
+    name if a file has an empty name.)
+    """
+    # For simplicity, encode with the original function, and then
+    # replace any 'filename="<key>"' with 'filename=""'. This isn't
+    # entirely robust, but is sufficient for testing use.
+    encoded = django.test.client.encode_multipart(boundary, data)
+    re_keys = r"|".join(re.escape(key) for key in data.keys())
+    return re.sub(
+        rb'filename="(%s)"' % re_keys.encode("ascii"),
+        b'filename=""', encoded)