supertest_test.js

// npm install chai express supertest
var fs = require('fs')
var expect = require('chai').expect

var express = require('express')
var supertest = require('supertest')

describe('file sending', function() {
    doTest('image/png') // works
    doTest('audio/mpeg') // fails

    function doTest(contentType) {
        it('works for ' + contentType, function(done) {
            var exampleFile = 'README.md'
            var expectedBuffer = fs.readFileSync(exampleFile)

            var app = express()
            app.get('/test', function(req, res) {
                res.set('Content-Type', contentType)
                res.download(exampleFile)
            })

            supertest(app).get('/test')
                .expect('Content-Type', contentType)
                .expect(200)
                .end(function(err, res) {
                    expect(Buffer.compare(expectedBuffer, res.body)).to.equal(0)
                    done()
                })
        })
    }
})