Feature/contentdisposition

I use a java servlet as a server that takes post request with images. It uses Apache Commons FileUpload. Multiple java libraries depend on this lib. Currently this lib does not support a request like

```
Content-Disposition: attachment; name="file"; filename="basketball.png"
Content-Type: image/png
```

Instead it only accepts requests like

```
Content-Disposition: form-data; name="file"; filename="basketball.png"
Content-Type: image/png
```

With my addon, we are be able to overwrite the default `attachment` with `form-data`. Default remains `attachment`. A sample request would look like

```javascript
var request = require('superagent');
request
	.post('http://localhost:8080/upload')
	.attach('file',  '/path/basketball.png', 'basketball.png', 'form-data')
	.end(function(err, res) {
		if (!err && res.ok) {
			console.log('yay got ' + JSON.stringify(res.body));
		} else {
			console.log('Oh no! error ');
		}
	});
```