ctx.body assignment of stream not working
Koa: 2.13.4
Node: 14.18.0

```javascript
ctx.attachment('logo.png');
ctx.body = fs.createReadStream('static/logo.png');
```

It returned text of structure stream instead of binary image content:
```
StatusCode        : 200                                                                                                                                                                              StatusDescription : OK                                                                                                                                                                               
Content           : {137, 80, 78, 71...}
RawContent        : HTTP/1.1 200 OK
                    Transfer-Encoding: chunked
                    Content-Disposition: attachment; filename="logo.png"
                    Connection: keep-alive
                    Keep-Alive: timeout=5
                    Content-Type: image/p...
Headers           : {[Transfer-Encoding, chunked], [Content-Disposition, attachment; filename="logo.png"], [Connection, keep-alive], [Keep-Alive, timeout=5]...}
RawContentLength  : 1544
``` 
