re: Proposal for sending multiple files via XMLHttpRequest.send()

I'd like to revive the "Proposal for sending multiple files via
XMLHttpRequest.send()" thread
started by Jian Li back in September.

As pointed out on that thread, sending a JS array of strings and File
references isn't going
to fly due to an array of strings already having meaning.

That thread concluded with Jonas Sicking saying:

> I definitely think that we need to add some way of sending a stream
> that is a concatenation of strings, binary data, and files, without
> requiring the files to be read.

What should this look like?  Should there be a new global constructor for an
object that supports methods like appendText and appendFile?  Like this:

var data = new UploadData;
data.appendText("foo");
data.appendFile(fileRef);
xhr.send(data);

I like the idea of creating a primitive that does not enforce any encoding.

Thoughts?
-Darin