[fix] Download file
## Describe the bug

**Node.js version:** 16

**OS version:** docker (alpine)

**Description:** WhenI go to http://locahost:9000/test, a download of /app/file.xml should be initiated in my browser.
## Actual behavior

I don't get the file content but the string "OK" (same behavior with another file or path)
```shell
$ curl http://localhost:9000/test
OK
```

```shell
$ curl -I http://localhost:9000/test
Content-Disposition: attachment; filename="numeezy.com.mobileconfig"
Content-Type: application/xml
Date: Sat, 03 Sep 2022 19:55:26 GMT
Connection: keep-alive
Keep-Alive: timeout=5
```


## Code to reproduce

```js
const Koa = require('koa');
const Router = require('koa-router');
const app = new Koa();
const router = new Router();


router.get('/test', (ctx) => {
	ctx.response.attachment('/app/file.xml');
	ctx.response.set('Content-Type', 'application/xml');
	ctx.response.status = 200;

});


app.use(router.routes());
app.use(router.allowedMethods());

app.listen(process.env.PORT || 9000);
```

Thanks for your help
