When overriding Zend_Controller_Action :: initView the viewSuffix will be overriden if you change it within the initView method.

Take the following code:

<?php

require_once 'Zend/Controller/Action.php';

class Sanmax_Controller_Action extends Zend_Controller_Action
{
    public function initView() 
    {
        if (null !== $this->view) {
            return; 
        }

        require_once 'Sanmax/View/Smarty.php';
        $config = Zend_Registry::get('config');

        $this->view = new Sanmax_View_Smarty($config->smarty->asArray());
        $this->viewSuffix = 'html';
                
        return $this->view;
    } 
}

Zend_Controller_Action will request the viewSuffix before it calls initView, and thus the viewSuffix will be reset to 'phtml'