Does not recognize $ _FILES codeigniter

I'm trying to send an image using postman and working with codeigniter REST API

test code:

public function profile_post(){
        $file = $_FILES['userfile']['name'];
           $this->response(array('message' => $file),200);
   }

postman output:

{
    "message": null
}

Even though I'm sending you a (userfile,name) image by postman using form-data does anyone know why it does not show me anything?