modules/document/document.item.php에서  썸네일 부분을 아래부분으로 교체하시면 됨

        function thumbnailExists($width = 80, $height = 0, $type = '') {
            if(!$this->document_srl) return false;
            if(!$this->getThumbnail($width, $height, $type)) return false;
            return true;
        }

        function getThumbnail($width = 80, $height = 0, $thumbnail_type = '') {
            if(!$this->document_srl) return;
            if(!$height) $height = $width;            

            // 문서 모듈의 기본 설정에서 Thumbnail의 생성 방법을 구함

            if(!in_array($thumbnail_type, array('crop','ratio'))) {
                $config = $GLOBALS['__document_config__'];
                if(!$config) {
                    $oDocumentModel = &getModel('document');
                    $config = $oDocumentModel->getDocumentConfig();
                    $GLOBALS['__document_config__'] = $config;
                }
                $thumbnail_type = $config->thumbnail_type;
            } 

            // 문서의 이미지 첨부파일 위치를 구함

            $document_path = sprintf('./files/attach/images/%d/%d/',$this->get('module_srl'), $this->get('document_srl'));
            if(!is_dir($document_path)) FileHandler::makeDir($document_path); 

            // 썸네일 임시 파일명을 구함

            if($width != $height) $thumbnail_file = sprintf('%sthumbnail_%dx%d_%s.jpg', $document_path, $width, $height, $thumbnail_type);
            else $thumbnail_file = sprintf('%sthumbnail_%d_%s.jpg', $document_path, $width, $thumbnail_type); 

            // 썸네일이 있더라도 글의 수정시간과 비교해서 다르면 다시 생성함

            if(file_exists($thumbnail_file)) {
                $file_created_time = date("YmdHis",filemtime($thumbnail_file));
                $modified_time = $this->get('last_update');
                if($modified_time > $file_created_time) @unlink($thumbnail_file);
            } 

            if(file_exists($thumbnail_file)&&filesize($thumbnail_file)<1) return; 

            // 썸네일 파일이 있으면 url return

            if(file_exists($thumbnail_file)) return Context::getRequestUri().$thumbnail_file; 

            // 생성 시작

            FileHandler::writeFile($thumbnail_file, '', 'w'); 

            // 첨부파일이 있는지 확인하고 있으면 썸네일 만듬

            $oFile = &getModel('file');
            $file_list = $oFile->getFiles($this->document_srl);
            if(count($file_list)) {
                foreach($file_list as $file) {
                    if($file->direct_download!='Y') continue;
                    if(!preg_match("/(jpg|png|jpeg|gif)$/i",$file->source_filename)) continue; 

                    $filename = $file->uploaded_filename;
                    if(!file_exists($filename)) continue; 

                    FileHandler::createImageFile($filename, $thumbnail_file, $width, $height, 'jpg', $thumbnail_type);
                    if(file_exists($thumbnail_file)) return Context::getRequestUri().$thumbnail_file;
                }
            } 

            // 첨부파일이 없으면 내용에서 추출

            $content = $this->get('content'+ '); 

            $target_src = null;
            preg_match_all("!http:\/\/([^ ^\"^']*?)\.(jpg|png|gif|jpeg)!is", $content, $matches, PREG_SET_ORDER);
            for($i=0;$i<count($matches);$i++) {
                $src = $matches[$i][0];
                if(preg_match('/\/(common|modules|widgets|addons|layouts)\//i', $src)) continue;
                else {
                    $target_src = $src;
                    break;
                }
            } 

            if($target_src) {
                $tmp_file = sprintf('%sthumbnail_%d.tmp.jpg', $document_path, $width);
                FileHandler::getRemoteFile($target_src, $tmp_file);
                FileHandler::createImageFile($tmp_file, $thumbnail_file, $width, $height, 'jpg', $config->thumbnail_type);
                @unlink($tmp_file);
                return Context::getRequestUri().$thumbnail_file;
            } 

            FileHandler::writeFile($thumbnail_file,'');
            return;
        }