.plain.html requests not correctly handled for local content files #2732

Description
when creating local html documents under the cloned aem content directory, .plain.html requests to these files are not served, resulting in 404s.

To Reproduce
Steps to reproduce the behavior:

1. run aem content clone --all
2. create a new file at content/nav.html and add nav markup to the file to it
3. run aem up
4. make a request to nav.plain.html and note that it 404s

Expected behavior
the plain html should be served, as it would be once the content is pushed and previewed

Version:
16.19.8