rocketchat: Ignore rocketchat attachments without types.

Not sure where those come from since we discovered this with production
data.

There only was a single instance of this in my entire batch of data in
an old message from the time when we started using Rocket.Chat. This
might be an old issue or it might require some special settings that
were later changed.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

zerver/data_import/rocketchat.py

@@ -370,6 +370,10 @@ def process_message_attachment(
         logging.info("Skipping unknown attachment of message_id: %s", message_id)
         return "", False
 
+    if "type" not in upload:  # nocoverage
+        logging.info("Skipping attachment without type of message_id: %s", message_id)
+        return "", False
+
     upload_file_data = upload_id_to_upload_data_map[upload["_id"]]
     file_name = upload["name"]
     file_ext = f'.{upload["type"].split("/")[-1]}'