File upload file_name not returning any values

PHP Code:

$this->load->helper(array('form', 'url'));
  
  $config['upload_path'] = 'uploads/file/';
  $config['allowed_types'] = 'xlsx|pdf|txt';
  $config['max_size'] = '5000';
  $this->load->library('upload');
  $this->upload->initialize($config);
$file = $this->upload->data();
$file_name = $file['file_name']; 


Everything is working fine and my uploaded file can move to it's path but i want to know why this code is not working.It is also indicated in the user guide. Even the all of the data from the uploaded file returns no value(like raw_name, file_ext). I searched all over the web and i can't find any answers to my problem.
I'm using Wampserver 2.5. Php 5.5.12.