Dot in path makes prettier not recognise Markdown files

This one had me pulling my hair out for a while. I'm trying to format a Markdown file in a subfolder, e.g.,

cat 1402.00708/article.md

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

prettier, however, doesn't do anything:

prettier 1402.00708/article.md --prose-wrap=always

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

Long story short, the culprit is the dot . in the folder name; prettier apparently doesn't recognize the file as Markdown then.

Reproduce the behavior with the shown prettier 1402.00708/article.md --prose-wrap=always command and compare it with a path without a dot. Then trace Prettier's file-type recognition for dotted paths; no source file or test is identified in the report, so locate the relevant entry point from the reproduction. Done means Markdown files in dotted directories are recognized and prose wrapping works.