docs: function sendAttachment (attachment, recipient) {
    return new Promise (async (resolve, reject) => {
      if (!attachment) {
        reject('attachment object required');      
      }
      
      let payload = new util.AttachmentPayload(attachment, recipient);
      
      if (!payload) reject('URL or file required');
      
      try {
        let response = await this.callSendApi(recipient, payload);  
        resolve(response);
      } catch (e) {
        reject(e);
      }
    });
  }