如何打开下载的文件？

您可以尝试下面在Google Chrome 38.0.2125.111 m（64位）中经过测试的示例代码

chrome.downloads.onChanged.addListener(function (detail){
console.log("Detail",detail); //Save log for debugging

    //if file download finished
   if(detail.state.current == "complete"){

       var downloadId = detail.id; //Download ID

       console.log("Download ID",downloadId); //Save log for debugging
       /*
       Do Something
        */

   }
});