How to add custom filters on the category page in Magento

Directory file path:   app/code/core/Mage/Catalog/Model/Category.php

public function getProductCollection()
    {
        $collection = Mage::getResourceModel('catalog/product_collection')
            ->setStoreId($this->getStoreId())
            ->addCategoryFilter($this)
->addAttributeToFilter('featured', array('value'=>'1'));
        return $collection;
    }