A discussion board forum is an area where participants discuss a topic or a group of related topics. Within each forum, users can create multiple threads. A thread includes the initial post and all replies to it. You can create forums and threads to organize discussions into units or topics relevant to your course.
If you want to get your students to submit content to a forum containing a mash up (eg to use Kaltura to record or upload a video), you will need to make sure that Allow File Attachments is ticked when you create your forum. If it isn’t, the Mashups dropdown won’t be available to anyone composing a thread or a reply.
We’ve reported this behaviour to Blackboard, and have been told this is by design.