Add FileList constructor

Split out from https://www.w3.org/Bugs/Public/show_bug.cgi?id=17125 :

The

<input type="file" name="something" />

element has always been readonly (you can't change value) for security purposes.
That makes sense as a string could always be manipulated by JS.

The File object on the other hand should be "safe", as in

• Only the browser can create one on user request (ie. by file input or drag and drop)
• JS cannot change it in any way

So it should not be a security issue to allow assignment to a file input.
For example:

<input type="file" name="something[]" onchange="splitFiles();" id="in1" multiple="multiple" />

function splitFiles() {
  var in = document.getElementById('in1');
  //create single inputs
  for(var i=0;i<in.files.length;i++) {
        var newEl = document.createElement('input');
        newEl.name = "something[]";
        newEl.id = '...';
        newEl.type = "file";
        newEl.value = in.files[i]; // <-- assignment of file object
        document.appendChild(newEl); //add for upload
        //create some UI to remove this input
        //...
    }
    //remove the multi-input
    in.parent.removeChild(in);
}

This code would split a multi-file-input back to multiple single-file inputs (that way applications with a legacy backend that only supports the classical way could also support multi-file-inputs).

Another use would be with File objects obtained by drag and drop.
Right now those require XHR to upload. Allowing to make them a selection of a file input could simplify some situations as well as provide richer UI (when showing a traditional file input together with a drop area those would stay in sync).

まず、FileList constructor のリクエストと、issue に記載された file-input、File-object、drag-and-drop、XHR の動作を確認します。関連する仕様テキストやテストを探す前に、意図する代入セマンティクスと完了基準を定義してください。issue にはリポジトリのファイルもテストのエントリポイントもありません。