Optional file upload

first question:

Why the file upload in COdeigniter is not optional so that I need to make a trick?

Second one:

In this code below If I select a file it says "You did not select a file to upload." and doesn't upload.
I have title,text,date and file fields in the form.

If I don't select a file there is no problem. Can you help me solve that?

PHP Code:

public function create()
    {
        $this->load->helper('form');
        $this->load->library('form_validation');

        $this->form_validation->set_rules('title', 'Başlık', 'trim|required');
        $this->form_validation->set_rules('text', 'Yazı', 'trim|required');
        
        $page="create";
        
        if ($this->form_validation->run() === FALSE)
        {
            $this->show($page);
        }
        else
        {
            $title=$this->input->post('title');
            $text=$this->input->post('text');
            $date=$this->input->post('date');
            $f=$this->input->post('file');
            //$this->load->library("uploader");
            if ($_FILES['dosya']['name'] != ""){
                $this->uploadfile();
            }else{
                $message="added without file successfully";
                $data['title']="Makale Listesi";
                $data['list']=$this->makale_model->listing(0);
                $data['page']=$page;
                $data['message']=$message;
                $this->show("modify",$data);
            }
            
            
            //$this->makale_model->insert($title,$text,$f,$date);
            //$f=$data['']
        }
    }

private function uploadfile()
    {
        $config['upload_path'] = base_url().'/files/makaleler/';
        $config['allowed_types'] = 'gif|jpg|png|bmp|jpeg|pdf|doc|docx|ppt|pptx';
        //$config['max_size']    = '100';
        //$config['max_width']  = '1024';
        //$config['max_height']  = '768';

        $this->load->library('upload', $config);        
        
        if ( ! $this->upload->do_upload())
        {
            $error = array('error' => $this->upload->display_errors());

            $this->show("create",$error);
        }
        else
        {
            $data = array('upload' => $this->upload->data());
            $message="added successfully with file";
            
            /*$data['title']="Listing";
            $data['list']=$this->makale_model->listing(0);
            $data['page']=$page;
            $data['message']=$message;
            $this->show("list",$data);
            */
            $message=urlencode($message);
            //redirect("makale/index?message=".$message,"refresh");
            $this->show("modify",$data);
        }
    }