Change allowed file extensions

Observer.php

<?php
public function changeAllowedFileExtensions(Varien_Event_Observer $observer) {
    $file = $observer->getEvent()->getFile();
    
    // Add new '.mp4' file extension to the list
    $allowedExtensions = $file->getData('allowed_extensions');
    $allowedExtensions[] = 'mp4';
    $file->setData('allowed_extensions', $allowedExtensions);
    
    // Or, ignore default list and only allow '.pdf' files
    $file->setData('allowed_extensions', array('pdf'));
    
    return $this;
}