support binding to HTMLInputElement.files property

[rafaelw]

[rafaelw]

So the files idl property of input is a FileList IDL object.

Here's how I would imagine this working

<input type="file" files="{{ inputFiles }}">

Would bind the value.files FileList object to the inputFiles property of the model in scope. If model.inputFiles has an existing value, it will be over-written at the time of binding (assigning to the files property of input is ignored).

The binding will then simply ensure that model.inputFiles points to the same FileList object which input.files points to (and that a polymer element may observe when that FileList object change by observing the change to the model property).