wx.downloadFile下载文件bin格式？

微信小程序下载文件格式为bin格式，请问是什么原因呢？

    wx.downloadFile({

            url: e.currentTarget.dataset.url,

            success: (res) => {

                console.log(res)

                let tempFilePath = res.tempFilePath;

                wx.openDocument({

                    filePath: tempFilePath

                })

            },

            fail: () => { },

            complete: () => { }

        });