I'm the admin for a large media intensive site. We've done some experimentation with our video MIME types. We've alternated between forcing the download using Content-Disposition: attachment + Content-type: application/octet-stream and actually returning the proper MIME type.
We've had complaints either way we set it up. With the proper MIME type many people have broken or incompatible browser plugins that try to play the content, but fail due to either format incompatibility or other issues. This leaves us with support emails trying to help customers configure their broken browsers. On the other hand, not using the proper MIME types upsets people who like to click on a file and have the plugin launch shortly thereafter.
We've seen maybe 20% fewer complaints when forcing the browser to download the file, so we're pretty sure we're going to stick with that, but I'd like to hear what other people think about this, since it seems like a real gray area.
For the record our file formats are WMV, XviD, QT mpeg4 mov, and Ipod m4v along with streaming flash.