Output as JPG sprite

Is there a specific parameter (with Gulp) to output as a `.jpg`? It seems that setting the `name` as something like `name: sprite.jpg` will output a file called `sprite.jpg`, but  the file is still encoded as a `.png`.

I'm currently using https://www.npmjs.org/package/gulp-gm to convert the output as a true `.jpg`, but was wondering if there was a way to do it without that extra step.
