Unable to upload files with 'file' form field, Ajax and AHAH helper modules

I'm writing a Drupal 6 module from scratch that needs uploading files in Ajax and I'm apparently hit by an incompatibility between Ajax, AHAH helper and 'file' type forms fields.

Here is the code that reproduces the issue:

function cdp_perm() {
	return array('access CDP application form');
}

function cdp_menu() {

	$items = array();

	$items['cdp/application'] = array(
		'title' => t('Application form'),
		'page callback'    => 'drupal_get_form',
		'page arguments'   => array('cdp_application_form'),
		'access callback'  => TRUE,
		'access arguments' => array('access CDP application form'),
		'type' => MENU_CALLBACK
	);

	return $items;
}

function cdp_application_form($form_state) {

	$form=array();

	ahah_helper_register($form, $form_state);

	$form['#ajax'] = array('enabled' => TRUE);

	$form['#attributes']['enctype'] = 'multipart/form-data';

	$form['visual'] = array(
		'#type' => 'fieldset',
		'#title' => t('Visual of the project'),
		'#prefix' => '<div id="visual-wrapper">',
		'#suffix' => '</div>',
		'#tree'   => TRUE,
	);

	$form['visual']['file'] = array(
		'#type' => 'file',
		'#title' => t('File'),
	);

	$form['visual']['upload'] = array(
		'#type'  => 'submit',
		'#value' => t('Upload'),
		'#submit' => array('cdp_uploadhack_submit'),
	);

	$form['submit'] = array('#type' => 'submit',
							'#value' => t('Save'),
							);
	return $form;
}

function cdp_application_form_submit($form, &$form_state) {
  drupal_set_message(t('You have successfully submitted your project.'));
}

function cdp_uploadhack_submit($form, &$form_state) {
	$form_state['rebuild'] = TRUE;

	if($file=file_save_upload('visual')) {
		drupal_set_message(t('upload ok'));
	}
	else {
		drupal_set_message(t('Nothing uploaded'), 'warning');
	}


}

The upload form is broken, you get the extroardinary helpful error message "ajax.module: An unknown error has occurred.".

If you comment « $form['#ajax'] = array('enabled' => TRUE); », it just work but Ajax is indeed not supported anymore for error reporting.

It look related to http://drupal.org/node/399676 but the work-around does not work.

If anyone know how to fix it, I would be thankful.

Regards,