File input field support

[jgaskins]

I was trying to add file upload support to a Clearwater app (it's using opal-browser for DOM interactions now), but I noticed that Browser::DOM::Element::Input doesn't support <input type="file">. I tried monkeypatching it in so I could submit it as a PR, but wrapping the native JS FileList and File objects were a bit more than my head was willing to process at the time. Any ideas on how to go about adding this?

[jgaskins]

Though probably understandable from context above, I don't think I said it well: what I mean is, for <input type="file">, I'd like to add a Ruby accessor for the files property.

[hmdne]

I am interested in implementing that