🐛 Bug Report

Let's say you have a file Overview.mdx and reference it from another document.

[Overview](./Overview) <-- fails silenty at runtime (leads to 404)
[Overview](./Overview.md) <-- fails at build time correctly because there is an .md file not .mdx
[Overview](./Overview.mdx) <-- works great

Have you read the Contributing Guidelines on issues?

Sure

To Reproduce

1. Use base path in your docusaurus config
2. Create 2 files
3. Reference one file from another without file extension

Expected behavior

That must be caught by the broken link handler.

(Write what you thought would happen.)

Actual Behavior

Links lead to 404 and I don't know about it until test it at runtime.

Your Environment

• Docusaurus version used: 2.0.0-alpha.62

Reproducible Demo

You can use docusaurus repo and reproduce locally.

Replace some relative link, for example here with a link without .md extension.