public function getThumbMime($revision = null)
	{
		// Without revision, use current info
		if (!isset($revision)) {
			return (isset($this->info['thumbmime']) ? $this->info['thumbmime'] : '');
		}

		// Obtain the properties of the revision
		if (($info = $this->getRevision($revision)) === null) {
			return null;
		}

		// Check for thumbnail MIME type
		return (isset($info['thumbmime']) ? $info['thumbmime'] : '');
	}