<?php
namespace EddcaponeMyExtensionController;

/**
 * CustomController
 */
class CustomController extends TYPO3CMSExtbaseMvcControllerActionController
{

    /**
     * action list
     *
     * @return void
     */
    public function confirmAgbAction()
    {
        echo "<p>HALLO WELT</p>";
    }
}

<?php
if (!defined('TYPO3_MODE')) {
    die('Access denied.');
}

TYPO3CMSExtbaseUtilityExtensionUtility::configurePlugin(
    'Eddcapone.' . $_EXTKEY,
    'Myfilelist',
    array(
        'Category' => 'list,show',
        'File' => 'show',
        'Download' => 'download',
        'Custom' => 'confirmAgb'
    ),
    // non-cacheable actions
    array(
        'Category' => 'list,show',
        'File' => 'topFive',
        'Download' => 'download',
        'Custom' => 'confirmAgb'
    )
);

<f:link.action controller="Custom" action="confirmAgb" pluginName="Myfilelist" class="mbButton">Download</f:link.action>

plugin.tx_yourextensionmvc.callDefaultActionIfActionCantBeResolved = 1