This function takes the same arguments as toDataURL(), plus an
additional 'name' argument. It produces the same result as
toDataURL(), except that it returns the result as a File object rather
than as a data-url encoded string. This can then be directly sent
using XMLHttpRequest.
I think it would make more sense to have an actual type for binary data
(for example along the lines of my proposal on public-script-coord and
es-discuss) and enable getting one from <canvas> and sending via XHR.
How about just overloading xhr.send() to handle a <canvas> element ?
