It's not actually clear what file extension you're supposed to use for textual wasm files. The Sublime plugin claims this language is called "WAST" and as such suggests .wast. I guess I'll go with that. MDN calls the format "WAT". I wonder if there's subtle commentary there.
It's also a *little* nonobvious how to assemble and run this stuff. Wasm was originally intended to be loaded from JavaScript. Both MDN and help on mastodon² suggest a toolset named WABT.
² https://mastodon.social/@mcc/114581638222018210