"application/octet-stream" ???

My installation of Avast (Free) interferes with viewing/hearing web media such as on YouTube. After clicking successfully (most of the time, but not always) subsequent downloads get a prompt that I am downloading an “application/octet-stream”. I have no idea what that is but it definitely isn’t what I want, nor what I experienced before installing Avast. The only solution I’ve found is to temporarily disable Avast. Is there something else I can do? I’ve got the most recent installation of Avast. I generally use Firefox, but the same thing happens with IE.

Suggestions?