Displaying a string representation of an image on client side using express

Im using jsftp in an express application to serve a string representation of an image from a FTP connection, which is working fine. However, the image does not display when sending it to the client:

router.get('/', (req, res, next) => {
  let str = ''
    ftp.get('/uploads/test.png', function (err, socket) {
      if (err) return
      socket.on('data', function (d) { str += d.toString() })
      socket.on('close', function (hadErr) {
        if (hadErr) {
          console.error('There was an error retrieving the file.')
        }
        res.writeHead(200, { 'Content-Type': 'image/png' })
        res.end(str, 'binary')
      })
      socket.resume()
    })
})

Any help is appreciated, thanks!