html5 file.slice only works first time

html5 file.slice only works first time

I'm trying to stream a file to the server
I've gotten this far http://jsfiddle.net/pu4Cz/2/
loadPart: function() {
var start = this.current() * this.bufferSize;
var blob = this.file.slice(start, this.bufferSize);
this.done = blob.size != this.bufferSize;
this.reader.readAsDataURL(blob);
}
The first slice works, but the next return size zero Try with a local file
bigger than bufferSize