[whatwg] Make files attribute of the input element writable

[Nico Weber]

> But, by the same token, XHR upload of FormData[1] already serves this use case in a more flexible way. You can even make a FormData from the contents of an html form and then add additional File objects. Making the change would (afaict) not serve any new use cases. At most it would be a convenience.

Agreed, it's a convenience, the same could be do with a sync XHR
followed by a location.reload(). The readonly restriction exists for
historical reasons, and dropping it seems like a small change that
makes the web platform more usable.

Nico