Use original filename in node sample post callback.

This change enables saving the uploaded file with correct filename using the write method of `flow.write`.

```javascript
flow.post(req, function(status, filename, originalFilename, identifier) {
  if (status === 'done') {
    var stream = fs.createWriteStream(__dirname + '/../images/' + originalFilename);
    flow.write(identifier, stream);
    stream.on('finish', function() {
      console.log('Finished writing ' + originalFilename);
    });
  }
});
```