i don't think that SMF would support that
the forum could easily support it if you write a plugin for it (or use one that already exists, and I'm sure there are several)however, it's kinda pointless. you should link to large files instead of embedding them in the first place
Siren stated that the webm version of a gif was smaller in file size, and I believe with my past experiences with them they tend to be.
if webm's have sound then you could easily post screamers/other really loving annoying things that you don't want to hear