Extend MY_Form_validation not work

I'm locked for extend my form_validation rules...
In my controller I have a function article_new() who have this rules :

PHP Code:

$this->form_validation
    ->set_rules('titre','Titre','trim|required|xss_clean|encode_php_tags')
    ->set_rules('image','Image','callback_handle_upload')
    ->set_rules('tags','Tags','trim|required|xss_clean|callback_tags|encode_php_tags')
    ->set_rules('article','Article','trim|required|xss_clean|encode_php_tags'); 


Here my function handle_upload :

PHP Code:

   <?php
    function handle_upload()
    {
        if (isset($_FILES['image']) && !empty($_FILES['image']['name']))
        {
            
        }
        else
        {
            // throw an error because nothing was uploaded
            $this->form_validation->set_message('handle_upload', "Vous devez uploader une image");
            return false;
        }
    }
    ?>

My function works well but I want to move this out of my controller.
So I put the function in the MY_Form_validation library located in application/librairies/MY_Form_validation.php.
If I do this, and I remove "callback_" in my rules it doesn't work... and I don't know why
I work with the 2.2.6 version.