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/inbound.py

@@ -99,7 +99,7 @@ class AnymailInboundMessage(Message):
     def inline_attachments(self):
         """dict of Content-ID: attachment (as MIMEPart objects)"""
         return {unquote(part['Content-ID']): part for part in self.walk()
-                if part.is_inline_attachment() and part['Content-ID']}
+                if part.is_inline_attachment() and part['Content-ID'] is not None}
 
     def get_address_header(self, header):
         """Return the value of header parsed into a (possibly-empty) list of EmailAddress objects"""
@@ -299,11 +299,11 @@ class AnymailInboundMessage(Message):
         # some sort of lazy attachment where the content is only pulled in if/when
         # requested (and then use file.chunks() to minimize memory usage)
         return cls.construct_attachment(
-            content_type=file.content_type,
+            content_type=getattr(file, 'content_type', None),
             content=file.read(),
-            filename=file.name,
+            filename=getattr(file, 'name', None),
             content_id=content_id,
-            charset=file.charset)
+            charset=getattr(file, 'charset', None))
 
     @classmethod
     def construct_attachment(cls, content_type, content,