[FileAPI] Make input[type=file] assignable for File objects (#24)

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

The
```html
<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:
```html
<input type="file" name="something[]" onchange="splitFiles();" id="in1" multiple="multiple" />
```
```js
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).

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/FileAPI/issues/24