I want to try to answer this... It's quite interesting that 2 years and 2 months later I can not figure it out how to easily solve the same problem.
I'm trying to get the attachment from a response like:
...
headers:     { 'cache-control': 'no-cache="set-cookie"',
       'content-type': 'multipart/related;boundary="----=_Part_61_425861994.1525782562904";type="application/xop+xml";start="";start-info="text/xml"',
...
body: '------=_Part_61_425861994.1525782562904\r\nContent-Type:
  application/xop+xml; charset=utf-8;
  type="text/xml"\r\nContent-Transfer-Encoding: 8bit\r\nContent-ID:
  \r\n\r\n....\r\n------=_Part_61_425861994.1525782562904\r\nContent-Type:
  application/octet-stream\r\nContent-Transfer-Encoding:
  binary\r\nContent-ID:
  \r\n\r\n�PNG\r\n\u001a\n\u0000\u0000\u0000\rIHDR\u0000\u0000\u0002,\u0000\u0000\u0005�\b\u0006\u0........binary....
I tried ws.js but no solution.
My solution:
var request = require("request");
var bsplit = require('buffer-split')
// it will extract "----=_Part_61_425861994.1525782562904" from the response
function getBoundaryFromResponse(response) {
    var contentType = response.headers['content-type']
    if (contentType && contentType.indexOf('boundary=') != -1 ) {
        return contentType.split(';')[1].replace('boundary=','').slice(1, -1)
    }
    return null
}
function splitBufferWithPattern(binaryData, boundary) {
    var b = new Buffer(binaryData),
        delim = new Buffer(boundary),
        result = bsplit(b, delim);
    return result
}
var options = {
    method: 'POST',
    url: 'http://bla.blabal.../file',
    gzip: true,
    headers: {
        SOAPAction: 'downloadFile',
        'Content-Type': 'text/xml;charset=UTF-8'
    },
    body: '<soapenv: ... xml request of the file ... elope>'
};
var data = [];
var buffer = null;
var filename = "test.png"
request(options, function (error, response, body) {
        if (error) throw new Error(error);
        if (filename && buffer) {
            console.log("filename: " + filename)
            console.log(buffer.toString('base64'))
            // after this, we can save the file from base64 ...
        }
    })
    .on('data', function (chunk) {
        data.push(chunk)
    })
    .on('end', function () {
        var onlyPayload = splitBufferWithPattern(Buffer.concat(data), '\r\n\r\n') // this will get from PNG
        buffer = onlyPayload[2]
        buffer = splitBufferWithPattern(buffer, '\r\n-')[0]
        console.log('Downloaded.');
    })
I am not sure it will works in most of the cases. It looks like unstable code to my eyes and so I'm looking for something better.