Preserve filename of inline attachments

For ESPs that support both content-id and filename,
don't drop the filename.

anymail/backends/mailgun.py

@@ -111,9 +111,14 @@ class MailgunPayload(RequestsPayload):
 
     def add_attachment(self, attachment):
         # http://docs.python-requests.org/en/v2.4.3/user/advanced/#post-multiple-multipart-encoded-files
-        field = "inline" if attachment.inline else "attachment"
+        if attachment.inline:
+            field = "inline"
+            name = attachment.cid
+        else:
+            field = "attachment"
+            name = attachment.name
         self.files.append(
-            (field, (attachment.name, attachment.content, attachment.mimetype))
+            (field, (name, attachment.content, attachment.mimetype))
         )
 
     def set_metadata(self, metadata):

anymail/backends/mandrill.py

@@ -118,10 +118,15 @@ class MandrillPayload(RequestsPayload):
         self.data["message"]["html"] = body
 
     def add_attachment(self, attachment):
-        key = "images" if attachment.inline else "attachments"
-        self.data["message"].setdefault(key, []).append({
+        if attachment.inline:
+            field = "images"
+            name = attachment.cid
+        else:
+            field = "attachments"
+            name = attachment.name or ""
+        self.data["message"].setdefault(field, []).append({
             "type": attachment.mimetype,
-            "name": attachment.name or "",
+            "name": name,
             "content": attachment.b64content
         })