Codeigniter form validation for file

I'm using codeigniter for my project and I need to read contents of files. So,I need validation to check whether the file is selected or not.

Here is my code in controller

$this->form_validation->set_rules(
  'estimation_file',
  'Project Estimation File',
  'required'
);
But while choosing a file it shows error saying - The Project Estimation File field is required