关于电视App升级更新问题，文件下载后读取提示失败

fuunction createDownload(){  
    var dtask = plus.downloader.createDownload(downloadApkUrl, {filename: '_doc/download/'});  
    dtask.addEventListener("statechanged", function(task, status){  
            var downloadedSize = 0,  
                totalSize = 0,  
                pre = 0;  

            switch(task.state){  
                case 1:  
                    console.log('开始下载...');  
                break;  
                case 2:  
                    console.log('连接服务器...');  
                    $("#loaded").show();  
                    mui("#progressbar").progressbar().show();  
                break;  
                case 3:  
                    totalSize = returnSize(task.totalSize);  
                    downloadedSize = returnSize(task.downloadedSize);  
                    $("#progressText").text(downloadedSize + "M / " + totalSize + 'M');  
                    pre = parseInt((downloadedSize/totalSize)*100);  
                    mui("#progressbar").progressbar().setProgress(pre);  
                break;  
                case 4:  
                    installWgt(task.filename);  
                break;  
            }  
    }, false);  
    dtask.start();  
}  

function installWgt(path){  
    plus.runtime.install(path, {}, function(){  
        plus.nativeUI.alert("应用更新完成！", function(){  
            plus.runtime.restart();  
        });  
    }, function(DOMException){  
        plus.nativeUI.alert(JSON.stringify(DOMException));  
    });  

    //plus.runtime.openFile(path);  
}

如果使用wgt文件更新可以安装成功
但是使用apk文件就提示读取文件失败，求大神告知