NodeJs Download Mongoose document as a file

I'm trying to trigger a file download from a mongoose document lookup however, I can't seem to return the returned data as a file.

As you can see i'm transforming some json data returned from the DB into a CSV, and then trying to send it to the client as a file type text/csv.

The data doesn't seem to be generate a file attachment when the response is sent.

Any suggestions or pointers in the right direction would be appreciated.

exports.download = function (req, res, next) {
  var dataSetId = req.params.id;

  DataContentModel.findById(dataSetId, function (err, doc) {
    if (err) return next(err);
    if (!doc) return res.send({'error': 'This record was not found.'});

    var keys = [];
    for(var k in doc.data[0]) keys.push(k);

    json2csv({data: doc.data, fields: keys}, function (err, csv) {
      if (err) console.log(err);

      res.setHeader('Content-Type', 'text/csv');
      res.setHeader('Content-Disposition', 'attachment; filename=csv.csv');
      res.send(csv);

    });
  });

};