Fix setting an attachment attribute on model

Right now we can only set the attachment attribute directly on a model (despite documentation says):
```php
$model->attachment = Input::file('attachment');
```
But it should be possible to set it using `fill()` or `setAttribute()` methods:
```php
$model->setAttribute('attachment', Input::file('attachment'));

// or

$model = new Model(['attachment' => Input::file('attachment')]);

// or

$model->fill(['attachment' => Input::file('attachment')]);
```