.mts files are excluded from coverage

[pmosconi]

Describe the bug

I have a node typescript module project and source files have extension .mts
Test coverage report doesn't include any source file, I verified this with both C8 and istanbul.
Tests run correctly regardless they have extension .mts or .ts.

Reproduction

Minimal example in https://github.com/pmosconi/vitest-issue

  Coverage enabled with c8

✓ tests/sample.test.mts (1)

Actual:
Test Files 1 passed (1)
Tests 1 passed (1)
Start at 12:51:48
Duration 1.17s (transform 455ms, setup 0ms, collect 64ms, tests 3ms)

% Coverage report from c8
----------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files | 0 | 0 | 0 | 0 |
----------|---------|----------|---------|---------|-------------------

Expected: src/index.mts to be in coverage report with coverage 100%

System Info

System:
    OS: Windows 10 10.0.19044
    CPU: (12) x64 Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz
    Memory: 16.80 GB / 31.76 GB
  Binaries:
    Node: 18.12.1 - C:\Program Files\nodejs\node.EXE
    npm: 8.19.2 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    vite: ^4.0.0 => 4.0.0
    vitest: ^0.25.7 => 0.25.7

Used Package Manager

npm

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