FileModel::onBeforeUpdate

static function onBeforeUpdate(Event $event)
{
	$parameters = $event->getParameters();
	$newFields = $parameters['fields'];
	$oldFields = static::getById($parameters['primary']['ID'])->fetch();
	foreach(static::$fileFieldNames as $name)
	{
		if(array_key_exists($name, $newFields) && $newFields[$name] != $oldFields[$name])
		{
			static::$filesToDelete[] = $oldFields[$name];
		}
	}
	return new MainEntityEventResult();
}