Can't use mjs config files

[mgrisole]

In a module package, I had to convert my postcss.config.mjs file to postcss.config.cjs (and specify its path in options postcss({config: {path: 'postcss.config.cjs', ctx: {}}})) to make it work.
It is not problematic but if that's intended (not to support mjs), could it be stated in the documentation?

[Kallekro]

I spent ~5 hours of my dear life because of this. No warnings, errors or anything to indicate this was an issue. Rollup was just not processing the CSS.

Thank you so much @mgrisole!