The correct mime type is vide/avi. As @Stomme poes says you should check you logs.
One common error is that the web-server does not have permissions to run the video so instead it returns 404 (on some servers). You should check that the video has permissions that are readable by you web server.
How are you setting the mime types? Do you have a .htaccess file or any rewriting code that may be running this afoul?