Feedback from developer team based on their checks is that the webm implementation may not be beneficial to all.
Specifically, this would require the use of a video tag and not an image tag to work. Unfortunately, video tags have several parameters to be defined but bbcode can only handle simple implementations. That said, it is likely restricted and will not work on Safari and IE browsers. The former is used by a very big audience, so this implementation is not going to benefit a lot of people.
gifv implementation was also considered, but we rather not have scripts running, which is required of this format.