If you haven't yet, make sure you read this stuff. My interpretation of it is that the content-disposition=attachment thing bypasses a lot of the other checks. It will still trigger a prompt, but would then probably give the user the checkbox to not prompt for that type in the future.

The article specifies a bit of a different syntax for the header than that in your post, though. It also seems to be different than the RFC. It's probably a typo on MS's part, but it seems to say that the actual header name is "Content-Disposition=attachment:" with the rest as the value. So, the header you're looking to use would be "Content-Disposition=attachment: filename=whatever.m3u". You might want to try that to see if that works. If it does, and it's indeed a different syntax than the RFC, that would be a bug worth knowing about.

edit : fixed link
-l0ser