So finally, I am here, with the code that does all the magical asynchronous upload of files. Sure it did take quite some time to get to the exact function, but all this time, the entire solution was staring right into my face !
In my previous blog post, I was exploring the possibility of allowing in YAHOO Mail Beta, the ability to select multiple files in the upload dialog, and uploading them simultaneously. Unable to find out what the "Attach" button exactly does and getting confused by the event handling model, I had decided to directly post to the given URL. That is when I saw that Yahoo Mail seems to use the YUI connection library. Ironically though, the file upload does not happen then.
All I would have to do now is to get the array of files selected for upload, and submit the form, one after another. That should quite do the job, and I can have my greasemonkey script ready !!