.mts for some time now, maybe overkill, but they are ESM files so it made sense. That said, I've had random success since moving to extended TypeScript language servers. In vanilla TypeScript it works as expected, but... outside of that it seems mildly random..ts resumes linting and things act like they should..mts and other variants in 4.5 beta and then 4.7 prod, I've been using this for some time, it's in the handbook. It is also included under TypeScript filetypes in WS, but to no avail.