Make isValid() check only for UploadedFile instances

This should fix: https://github.com/laravel/framework/issues/5455

Simply moved the `isValid` out so it only runs for uploaded files as the method only exists on the UploadedFile class, not the underlying File class.
The whitespace is wrong. It should be:
```php
if ($value instanceof UploadedFile && ! $value->isValid())
```