View

  echo $this--->Form->create(false, array(
      'type' => 'file', // <---- Important
      'url' => array(
          'admin' => true,
          'controller' => 'tab_trees',
          'action' => 'upload_json', $this->request->data['TabTree']['id']
      )
  ));

  echo $this->Form->input('file', array(
      'class' => 'form-control',
      'label' => false,
      'type' => 'file',  // <---- Important
  )); 

  echo $this->Form->end(array(
      'label' => __('Upload File'),
      'class' => 'btn btn-sm btn-primary'
  ));

Controller

public function admin_upload_json($id = null)
{
    debug($this->request->data);
    die;

    $this->TabTree->id = $id;
    if (!$this->TabTree->exists) {
        throw new NotFoundException(sprintf(__('Invalid %s'), __('tab tree')));
    }
}