public function bulk_product_upload_file()
{
    $location_of_bulk_product_files = LOCATION OF FOLDER;
    if ($this->is_post()):
        if (is_uploaded_file($_FILES['userfile']['tmp_name'])):
            $date_for_file = date("dmyHis");

            $config['upload_path'] = $location_of_bulk_product_files; //upload directory
            $config['allowed_types'] = 'xlsx'; // allowed file types
            $config['max_size'] = '100';
            $config['overwrite']  = TRUE;
            $config['remove_space']  = TRUE;
            $config['file_name'] = $date_for_file;
            $this->load->library('upload', $config);

            if($this->upload->do_upload()):
            // I have also tried if($this->upload->do_upload('userfile')): that resulted in the same error
                echo "file upload success";
            else:
               echo "file upload failed";
            endif;
        else:
            $this->session->set_flashdata('message_error','Please select a file to upload.');
            redirect('main/create_bulk_product');
        endif;
    else:
        $this->session->set_flashdata('message_error','Sorry! Somthing went wrong, please try again.');
        redirect('main/create_bulk_product');
    endif;
}