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

anymail/webhooks/sendgrid.py

@@ -181,12 +181,22 @@ class SendGridInboundWebhookView(AnymailBaseWebhookView):
             attachments = None
         else:
             # Load attachments from posted files
-            attachments = [
-                AnymailInboundMessage.construct_attachment_from_uploaded_file(
-                    request.FILES[att_id],
-                    content_id=attachment_info[att_id].get("content-id", None))
-                for att_id in sorted(attachment_info.keys())
-            ]
+            attachments = []
+            for attachment_id in sorted(attachment_info.keys()):
+                try:
+                    file = request.FILES[attachment_id]
+                except KeyError:
+                    # Django's multipart/form-data handling drops FILES with certain
+                    # filenames (for security) or with empty filenames (Django ticket 15879).
+                    # (To avoid this problem, enable SendGrid's "raw, full MIME" inbound option.)
+                    pass
+                else:
+                    # (This deliberately ignores attachment_info[attachment_id]["filename"],
+                    # which has not passed through Django's filename sanitization.)
+                    content_id = attachment_info[attachment_id].get("content-id")
+                    attachment = AnymailInboundMessage.construct_attachment_from_uploaded_file(
+                        file, content_id=content_id)
+                    attachments.append(attachment)
 
         default_charset = request.POST.encoding.lower()  # (probably utf-8)
         text = request.POST.get('text')