openDocument(item) {
    let url = `${item.fileDownloadUrl}`
    console.log(`下载路径：${url}`)
    this.System.showLoading('正在下载')
    if (['png', 'gif', 'jpg', 'jpeg'].indexOf(item.fileSuffix) != -1) {
        uni.downloadFile({
            url,
            success: (res) => {
                if (res.statusCode === 200) {
                    var filePath = res.tempFilePath;
                    this.System.storage.setItem(item.fileStorageName, filePath)
                    console.log(res)
                    // 预览图片
                    uni.previewImage({
                        urls: [filePath],
                        longPressActions: {
                            itemList: ['发送给朋友', '保存图片', '收藏'],
                            success: function(data) {
                                console.log('选中了第' + (data.tapIndex + 1) + '个按钮,第' + (data.index + 1) + '张图片');
                            },
                            fail: function(err) {
                                console.log(err.errMsg);
                                // 如果打开失败，则重新下载
                                this.openDocument(item)
                            }
                        }
                    })
                } else {
                    this.System.alert('文件下载错误')
                }
            },fail: res => {
                console.log(res);
            },
            complete: () => {
                this.System.hideLoading()
            }
        })
    } else {
        wx.request({
            url,
            header: {
                "content-type": "application/json",
                "token": this.System.storage.getItem('token')
            },
            method: "GET",
            responseType: "arraybuffer", //此处是请求文件流，必须带入的属性
            success: res => {
                console.log('res', res)
                if(res.statusCode === 200){
                    const fs = wx.getFileSystemManager(); //获取全局唯一的文件管理器
                    let filePath2 = wx.env.USER_DATA_PATH + '/' + item.fileDisplayName
                    console.log(filePath2);
                    fs.writeFile({
                        // 写文件
                        filePath: filePath2, // wx.env.USER_DATA_PATH 指定临时文件存入的路径，后面字符串自定义
                        data: res.data,
                        encoding: 'binary', //二进制流文件必须是 binary
                        success: fileRes => {
                            wx.openDocument({
                                // 新开页面打开文档
                                filePath: filePath2, //拿上面存入的文件路径
                                showMenu: true,
                                success: docRes => {
                                },
                                fail: (err) => {
                                    console.log(err)
                                    if (err.errMsg == 'openDocument:fail no third apps supported') {
                                        this.System.alert('您的手机不支持打开此文件，请先安装“WPS或腾讯文档”软件再打开')
                                    } else if (err.errMsg == 'openDocument:fail filetype not supported') {
                                        this.System.alert('您的手机不支持打开此文件，请先安装“WPS或腾讯文档”软件再打开')
                                    } else {
                                        // 如果打开失败，则重新下载
                                        this.openDocument(item)
                                    }
                                }
                            });
                        },
                        complete: res => {
                            console.log('res', res);
                        }
                    })
                } else {
                    this.System.alert('文件不存在')
                }
            },
            complete: () => {
                this.System.hideLoading()
            }
        })
    }
}

item 数据主要结构大致如下：
{
    fileDisplayName: "工程开工报审表.jpg"
    fileDownloadUrl: "xxx" // 文件请求的字节流方法
    fileStorageName: "d2ce05bb5bc60dad444812jpg"
    fileSuffix: "jpg"
    name: "工程开工报审表.jpg"
    outSystemId: null
    url: "xxx" // 文件请求的真是路径
}

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