We've been having that problem intermittently, and we're not sure why - checking the logs, it appears that some browsers, sometimes, are setting the mimetype of swf uploads to "application/octet-stream" instead of "application/x-shockwave-flash".  We've heard from folks for whom IE fails and Firefox/Mozilla work, and from others for whom IE works and Firefox/Mozilla fail.

We'll be deploying a new site in a week or so and do what we can to make sure it works with that one - at this point we're not spending much effort to maintain the current one.