File upload validation

[eluser]borgir[/eluser]
Hey there!
Sorry to bother you guys with the same issue...
I'm basing my validation on PistolPete's example and I'm always presented with the message:
"O campo Imagem é obrigatório." which means: "The image field is required."

Controller:

Code:

function enviar() {
        $this-> validate_form();
    }
    

     function validate_form() {
        
        $this->form_validation->set_error_delimiters('<p style="color:red;">', '</p>');

        $this->form_validation->set_rules('assunto', 'Assunto', 'trim|required|max_length[50]');
        $this->form_validation->set_rules('descricao', 'Descrição', 'trim|required');
        $this->form_validation->set_rules('userfile', 'Imagem', 'required|callback__do_upload');
        
        if ($this->form_validation->run() == FALSE) {
            $data['main_content'] = 'newsletter/newsletter_create_view';    
            $this->load->view('includes/template', $data);    
        } else {
            
        }
            
     }
    
    
     function _do_upload($file) {
        
        $config['upload_path'] = './uploads/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size']    = '3000';
        $config['max_width']  = '1024';
        $config['max_height']  = '768';

        $this->load->library('upload', $config);
            
        if (!$this->upload->do_upload())
        {
            $this->form_validation->set_message('_do_upload', $this->upload->display_errors());
            return FALSE;
        } else {
            return TRUE;
        }
    }

View:

Code:

<?php echo utf8_encode(form_error('assunto')); ?>
<?php echo utf8_encode(form_error('descricao')); ?>
<?php echo utf8_encode(form_error('userfile')); ?>

<form method="post" action="<?php echo site_url() . '/newsletter/enviar'; ?>" enctype="multipart/form-data">
    <input type="text" name="assunto" value="<?php echo set_value('assunto'); ?>" size="60" />
    <textarea name="descricao" rows="3" cols="74"><?php echo set_value('descricao'); ?></textarea>
    <input type="file" name="userfile" />
    <input type="submit" value="GUARDAR">
</form>

Can anyone give me a hint?
Thks a lot!!