public function save($destinationFolder, $newFileName = null){
$this->_validateFile();

if ($this->_allowCreateFolders) {
    $this->_createDestinationFolder($destinationFolder);
}

if (!is_writable($destinationFolder)) {
    // Put a mage::log() and check what is the folder magento is looking for.
    Mage::log($destinationFolder);
    throw new Exception('Destination folder is not writable or does not exists.');
}