在小程序端，.js文件中，使用以下代码下载文件

wx.showLoading({
  title: '下载中',
});
console.log("下载文件：",fileId);
wx.cloud.downloadFile({
  fileID: fileId,
  timeout: 30,
  success: (result) => {
    console.log("下载成功",result.tempFilePath);
  },
  fail: (res) => {
    console.log("下载失败：",res);
    wx.showToast({
      title: res.errMsg,
      icon:'error'
    });
  },
  complete: (res) => {
    wx.hideLoading();
  },
})

1结果：模拟器下载文件成功。

真机预览（开发版）和体验版本，都是提示：“cloud.downloadFile:fail undefined . downloadFile:fail timeout"

{message: "cloud.downloadFile:fail undefined . downloadFile:fail timeout", errCode: -1, errMsg: "cloud.downloadFile:fail undefined . downloadFile:fail timeout", line: 1, column: 234894, …}column: 234894errCode: -1errMsg: "cloud.downloadFile:fail undefined . downloadFile:fail timeout"line: 1message: "cloud.downloadFile:fail undefined . downloadFile:fail timeout"sourceURL: "https://lib/WACloud.js"stack: "R@https://lib/WACloud.js:1:234894↵@https://lib/WACloud.js:1:311533↵t@https://lib/WACloud.js:1:375↵u@https://lib/WACloud.js:1:623↵q@https://lib/WASubContext.js:1:70977↵@https://lib/WASubContext.js:1:71199↵C@https://lib/WASubContext.js:1:40676↵@https://lib/WASubContext.js:1:54498↵I@https://lib/WASubContext.js:1:54267↵@https://lib/WASubContext.js:1:54305↵f@https://lib/WASubContext.js:1:163627↵@https://lib/WASubContext.js:1:126859↵@https://lib/WASubContext.js:1:158959↵p@https://lib/WAServiceMainContext.js:1:399727↵@https://lib/WAServiceMainContext.js:1:387729↵_onNativeTimer@↵global code@"