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.