Buffer Schema Type in Mongoose: How to specify subtype

bintest.js

'use strict';

const mongoose       = require('mongoose');
const Schema         = mongoose.Schema;
const ObjectId       = Schema.Types.ObjectId;
const MongooseBuffer = mongoose.Types.Buffer;
const uuid           = require('node-uuid');

const db             = require('../lib/database');

const BinTestSchema = new Schema({
    _id: {
        type: Schema.Types.Buffer,
        default : () => {
            const buffer = uuid.v4(null, new Buffer(16));
            return new MongooseBuffer(buffer).toObject(0x03);
        },
        get: (buffer) => uuid.unparse(buffer),
        set: (string) => {
            const buffer = uuid.parse(string);
            return new MongooseBuffer(buffer).toObject(0x03);
        }
    } // results in: BinData(3,"2S2M2YPQRFGF6S5YTMDoOA==")
});

module.exports = db.model('BinTest', BinTestSchema);