How to access variables sent back into the FileReference object


Just trying to get my head round this. I have read in the flash help system that files uploaded using this method are sent via POST. I am trying to access a variable which is sent back by my php script after the file is uploaded. Normally if I was doing this using the LoadVars object then I could easily access this.

Anyone know how I can access the variable using this new method for uploading files? Does it get put into _root or does it go somewhere else?