var stream = require('koa-stream');
var path   = require('path');
var app    = require('koa');
app.use(function *() {
    yield stream.file(this, 'my-video.mp4', {root: path.join(__dirname, '/public')});
});

var stream = require('koa-stream');
var path   = require('path');
var app    = require('koa');
app.use(function *() {
    stream.buffer(this, new Buffer([1,2,3]), 'image/png', {allowDownload: true});
});