What you are trying to do would pose a huge security problem, and is therefore explicitly not allowed. Imagine a scenario such as where you tell someone that they are downloading happyBunnies.swf, when they are actually downloading maliciousVirus.exe .
If you find a way to do what you're asking, please report it as a security hole.
It's possible that I'm just misunderstanding the question, so please feel free to reply for a clarification of a less dangerous issue.