Security measures are often like that I find.

Flash has this concept called "cross domain policy", which basically means that you can't get Flash to read files from a server other than the one it's hosted on unless there's an explicit permission thingummy set up on it.

Which is all very sensible unless you're a contractor developing software for someone else's site at which point it bites you in the @ss every time! (The downloads in question fail silently, so you need to remember why - it won't tell you.)