Attach files via drag and drop or file input.

The element itself doesn't seem to care about file inputs. In dotcom we use a custom behavior that calls fileattachment.attach(files). I think we can add some simple documentation for this, but I wonder what you all think about having the element listening on file input change instead?

It is not too much to add the custom hook, but considering an actual file input would be needed for accessibility, I think it'd be good if it comes by default.

cc @github/web-systems