VS2017 & Web Essentials: How to minify .js and change filetype?

In Visual Studio 2017 & Web Essentials, how to minify and change file type of minified file on build?

For example: Input: helper.js Desired output on build: helper.min.xjs (...where .xjs is some other filetype we need for specific reasons)

I have this working, except that I can't change the filetype of minified file.

Bundleconfig.json doesn't accept the non-".js" filetype, and I get the following warning:

Any thoughts? I tried searching this but only came up with some Gulp&Node.js related answers.