Got Invalid PNG Data from getDownload API

[mikectint]

In our application I try to download an image attachment of a message in a messaging conversation, using JavaScript SDK of the API "GET /api/v2/downloads/{downloadId}".

let downloadId = "577048f050fe9354"; 
let opts = { 
  "contentDisposition": "", 
  "issueRedirect": true, 
  "redirectToAuth": true 
};
apiInstance.getDownload(downloadId, opts)
  .then((data) => {
    console.log(`getDownload success! data: ${JSON.stringify(data, null, 2)}`);
  })
  .catch((err) => {
    console.log("There was a failure calling getDownload");
    console.error(err);
  });

According to the documentation, if the request is successful it should return a JSON in the form of

{
    "url":""
}

However, it returns a string that looks neither binary nor base64 string. (it starts with

�PNG\r\n\n���\rIHDR��������\b���\tm\"H����PLTE��������������ȗ����̛����������������������Ҕ�����ccc��򧧧��배�yyyDDD�����ٍ��ppp���������QQQ---~~~������HHH���(((��YYY���999mmm\u000b\u000b\u000bVVVqpY]\\I����Û65*�⳧����r����թwv^+++��y���IH9**!�ɟ��i#\"e����ټ��������ˮ������ɬ��x���\u000b\u000bff[BA3UTC$dcN����IDATx��]i{����e�!�\\6�Zn�\u000b����3�Mv7����\t���ҁZ0y���;k#�WU]U]]�\n�q�g�q�g�q�g�q�g�q�g�q�

)

And when I save the data into a PNG file and open it with a image viewer, it shows the error that it does not support this file format.

My questions are:

1. What format is the return string?
2. How to convert the image into base64 string?