public function isValid()
	{
		// loop through all files
		foreach ($this->container as $file)
		{
			// return false at the first non-valid file
			if ( ! $file->isValid())
			{
				return false;
			}
		}

		// only return true if there are uploaded files, and they are all valid
		return empty($this->container) ? false : true;
	}