How can I overwrite Mage_Adminhtml_Block_Cache?

Question

I'm trying to add a button on the cache page, next to the "Flush Cache Storage" button. My approach is to overwrite the block "Mage_Adminhtml_Block_Cache". But I can't get it to work. Does anyone know what I'm doing wrong?

Mymodule/FlushImages/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Mymodule_FlushImages>
            <version>1.0.0</version>
        </Mymodule_FlushImages>
    </modules>
    <global>        
        <blocks>
            <adminhtml>
                <rewrite>
                    <cache>Mymodule_FlushImages_Block_Adminhtml_Cache</cache>
                </rewrite>
            </adminhtml>
        </blocks>                                                                                                   
    </global>
</config>

Mymodule/FlushImages/Block/Adminhtml/Cache.php

<?php

    class Mymodule_FlushImages_Block_Adminhtml_Cache extends Mage_Adminhtml_Block_Cache
    {
        /**
         * Class constructor
         */
        public function __construct()
        {
            $this->_addButton('flush_magento', array(
                'label'     => Mage::helper('core')->__('Flush Images'),
                'onclick'   => 'setLocation(\'' . $this->getFlushImagesUrl() .'\')',
                'class'     => 'delete',
            ));

            parent::__construct();
        }

        /**
         * Get url for delete Images
         */
        public function getFlushImagesUrl()
        {
            return $this->getUrl('*/*/flushImages');
        }
    }

Note: I did create the module config file in app/etc/modules.

Help is highly appreciated.

Answer 1

your line:

<adminhtml_cache>Mymodule_FlushImages_Block_Adminhtml_Cache</adminhtml_cache>

should be replaced with:

<cache>Mymodule_FlushImages_Block_Adminhtml_Cache</cache>

Answer 2

Finally I found a way - by using an model/observer:

etc/config.xml

<adminhtml>
    <events>
        <core_block_abstract_prepare_layout_before>
            <observers>
                <clearimages_core_block_abstract_prepare_layout_before>
                    <class>clearimages/observer</class>
                    <method>addButton</method>
                </clearimages_core_block_abstract_prepare_layout_before>
            </observers>
        </core_block_abstract_prepare_layout_before>
    </events>
</adminhtml>

Model/Observer

class Mymodule_FlushImages_Model_Observer
{
    public function addButton($observer) {

        $block = $observer->getEvent()->getBlock();
        if(get_class($block) == 'Mage_Adminhtml_Block_Cache') {
            $button = $block->addButton('clearimages', array(
                ...
            ));
        }
    }
}