无法创建FileList类型的对象(duck类型)

我想创建一个遵循以下接口的对象

interface FileList {
  getter File? item(unsigned long index);
  readonly attribute unsigned long length;
};

最后,我想把这个对象分配给 files input type=file HTML 要素

我写的代码是

let file1 = new File(["foo"],"foo.txt");
    let fl:FileList = {
      item: function(index){
        file1;
      },
      length: 1
    } as FileList;
...
imageInputNE.files = fl ;

TypeError: Failed to set the 'files' property on 'HTMLInputElement': The provided value is not of type 'FileList'.

我试着创造一个 typescript

class MyFileList{
  file:File;
  length=1;
  constructor(file:File){
    this.file = file;
  }

  item(index):File|null {
    return this.file;
  }

}

...
let file1 = new File(["foo"],"foo.txt");
    let fl:MyFileList = new MyFileList(file1);
imageInputNE.files = fl ;

如何创建类型的对象 FileList ?