If renpy cannot easily check if the folder has webms inside and not just pictures, then folder should start with "webm". Otherwise there are name, channel and loop. Spaces are separators, just like with usual images.
So it would be "image_name channel_name 1" folder name for looped movie, or "image_name channel_name" for non looped one. We could use a default channel, probably main_gfx_attacks since I use it everywhere if possible to not create new channels.
So it also could be image_name 1 for default channel + looped or just image_name for non looped + default channel.
As for webms names inside he folder, they are always "movie.webm" and optional "mask.webm".