mint dev incorrectly treats lowercase changelog.md as missing, while changelog.mdx works

[shushenghong]

Description

mint dev incorrectly reports a lowercase changelog.md page as missing, even though the file exists and is referenced in docs.json.

Changing the same file from changelog.md to changelog.mdx makes the route work immediately without changing the route in docs.json.

Reproduction

Project structure:

zh/rtc/ios/changelog.md

docs.json contains:

"zh/rtc/ios/changelog"

Then run:

mint dev

### Actual behavior

mint dev prints:

warning - "zh/rtc/ios/changelog" is referenced in the docs.json navigation but the file does not exist.

The route returns 404 locally.

### Expected behavior

mint dev should recognize lowercase changelog.md as a valid page file when it exists on disk and is referenced in docs.json.

### Workaround

Renaming the file from:

zh/rtc/ios/changelog.md

to:

zh/rtc/ios/changelog.mdx

fixes the issue immediately, with the same route:

/zh/rtc/ios/changelog

### Environment

- Mint CLI: 4.2.408
- Node.js: 20.19.6
- OS: macOS