And, your image doesn't work with Firefox.
Firefox does not support webm, since it is not widely used. I can't do anything about it
Maybe one day firefox will add webp support.
As for webm, it is not a image since you mentioned "Insert image" i thought you mistaken it for webp. and yes you are right webm is not supported.
Don't know if it will be supported in future or not, can't say anything about it at this point. SMF by default does not support webm(i think).
I will look into it, no promises though.