Show an error if there’s a file with no extension

[frankieroberto]

This aims to help catch a common error: people adding a markdown file for a post but forgetting to add the .md file extension.

If a file is found, the build will fail with an error like this:

[11ty] Wrote 0 files in 0.27 seconds (v3.1.5)
[11ty] Eleventy Fatal Error (CLI):
[11ty] Found file without extension: app/lung-health-check/2025/08/lung-health-check-prototype/test
[11ty] 
[11ty] Did you forget to add an .md to it?

This will be reported by GitHub when opening a pull request (as that does a test build), or locally in terminal.

Fixes #37

[edwardhorsford]

Very nice! how do we test this?

[frankieroberto]

@edwardhorsford you can test it locally (fetch this branch, then add a file with no extension).

Or to see it on GitHub I've just opened a demo PR here: #440

[edwardhorsford]

Great work!