由于文件夹是对象的集合，因此必须为每个对象添加一个索引.

Because Folder is a collection of objects, you have to add each one of them with an index.

var index = 0;
for(var pair of folderss){
    var folder = pair[key];
    formData.append("Folders[" + index + "].Id", folder.Id);
    index++;
}