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).

Inizia esaminando la richiesta relativa a FileList constructor e il comportamento di file-input, File-object, drag-and-drop e XHR descritto nell'issue. Definisci la semantica di assegnazione prevista e i criteri di completamento prima di individuare il testo pertinente della specifica o i test; l'issue non fornisce file del repository né punti di ingresso dei test.