Initiate file download with Koa

You should be able to simple set this.body to the file stream

this.body = fs.createReadStream(__dirname + '/test.zip');

then set the response headers as appropriate.

this.set('Content-disposition', 'attachment; filename=' + filename);
this.set('Content-type', mimetype);

For anyone else who sees this in the future, it's worth mentioning that there is a built in attachment method on the response object that you can use to set the Content-Disposition to attachment with a specified filename. So you can do this:

this.attachment('hello.txt')

And it would be the same thing as the following:

this.set('Content-disposition', 'attachment; filename=hello.txt')

---

For Koa 2:

ctx.attachment('hello.txt')

Bit of a different file download example with error checking, using Node with Koa 2. I am not sure if it is necessary to destroy the stream afterwards as described here https://github.com/jshttp/content-disposition#options

router.get('/downloads/:version/:file', async function(ctx) {
  const fileName = `${__dirname}/downloads/${ctx.params.version}/${ctx.params.file}`;
  try {
    if (fs.existsSync(fileName)) {
      ctx.body = fs.createReadStream(fileName);
      ctx.attachment(fileName);
    } else {
      ctx.throw(400, "Requested file not found on server");
    }
  } catch(error) {
    ctx.throw(500, error);
  }  
});

In browser sample: https://myserver.com/downloads/1.0.0/CoolAppFile.zip