저는 삽질해서 실현했습니다.

/modules/document/document.item.php

function getThumbnail( 부분

// 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;
 
                    // gif 対応　2008.07.09

                    //이부분을 추가했습니다.
                    $pathvar = pathinfo($filename);
                    if ($pathvar['extension'] == 'gif') {
                        $thumbnail_file = sprintf('%sthumbnail_%dx%d_%s.jpg', $document_path, $width, $height, $thumbnail_type);
                         copy($filename,$thumbnail_file);
                            //FileHandler::createImageFile($filename, $thumbnail_file, $width, $height, 'gif', $thumbnail_type);
                    } else {
                        FileHandler::createImageFile($filename, $thumbnail_file, $width, $height, 'jpg', $thumbnail_type);
                    }

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