I don't have the code for parsing and downloading attachments from the notification payload. But you can modify this code snippet to work with the notification payload.

public void read_message_store_sms() throws RestException, IOException{

        ListMessagesParameters parameters = new ListMessagesParameters();

        parameters.messageType = new String[] {"SMS"};

        parameters.dateFrom = "2023-05-01T00:00:00.222Z";

        

        var response = restClient.restapi().account().extension().messageStore().list(parameters);

        for (GetMessageInfoResponse record : response.records)

        {

            if (record.attachments != null)

            {

                for (var attachment : record.attachments)

                {

                    var fileName = "./src/test/resources/" + record.attachments[0].id + "_mms";

                    if (attachment.type.equals("MmsAttachment"))

                    {

                        if (attachment.contentType.equals("image/png"))

                            fileName += ".png";

                        else if (attachment.contentType.equals("image/jpeg"))

                            fileName += ".jpg";

                        else

                            fileName += ".xxx"; 

                    var res = restClient.restapi().account().extension().messageStore(record.id.toString()).content(attachment.id.toString()).get();

                    Path path = Paths.get(fileName);

                    Files.write(path, res);

                    }

                }

            }

        }

    }

All you need is to parse the payload, check the "attachments" then use the code above to download it.

Note: This code uses the RingCentral Java SDK.