Only when using FileReference.upload(), you will get
"progress" events from the flash player which you can use to track
the progress of your request. That means, only during File Upload,
you will be able to track how many bytes have been uploaded.
In all other cases, you just get to know if sending your
request has completed or not (by listening to the open event).
Anybody knows if this class has improved to solve my
problem?? Or is there any alternative way to send a multi-part
(The last time I read the SDK, there wasn't any way to send a
"multipart form".that's why I actually use this class to send a
multiform as data)