i don't think the forum supports movie formats, just mp3 (specifically, not just 'audio' as wmv or FLAC wont work, for those try soundcloud then link it on the forum).
for video, i use vimeo and youtube. let them handle the processing of the file you have to link to vimeo but the forum will allow you to display it directly in threads.
both video services should accept a variety of the common formats.
if you only have video and still have problems. you can download realplayer's free version , and extract the audio from any video as mp3, you can then attach that to the message/thread.