Welcome to the RingCentral Community

To send a fax with attachment is a base64 string using the RingCentral Node JS SDK, try this.

async function sendFax() {
  try{
    var endpoint = "/restapi/v1.0/account/~/extension/~/fax"
    var FormData = require('form-data');
    formData = new FormData();
    var body = {
      to: [{'phoneNumber': "RECIPIENT-NUMBER"}],
      faxResolution: 'High',
    }

    formData.append('json', new Buffer.from(JSON.stringify(body)), {
        filename: 'test.png',
        contentType: 'application/json'
        });

    var base64Data = "...JsHpb9we8DAfN9fUY+h9OL9b4NKMvqwAAAABJRU5ErkJggg=="
    var bufferValue = Buffer.from(base64Data,"base64");
    formData.append('attachment', bufferValue, 'image.png');
    var resp = await platform.post(endpoint, formData)
    var jsonObj = await resp.json()
    console.log("FAX sent. Current status: " + jsonObj.messageStatus + "/" + jsonObj.id)
  }catch(e){
    console.log(e.message)
  }
}

BTW, there is a current issue with receiving fax attachment in the sandbox environment. You will only get the cover page, no attachment pages but the fax status should be "Sent" though.