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I'm guessing write.as is supposed to support multiple languages, and only renders the markdown if you have .md as an extension.

Which is the exact opposite of what I expect and want -- the bare filename should render, and the .md extension should get the markdown source.

Agreed. Write.as is open source, you can file a ticket against WriteFreely on github

If the source can be different languages (like md, html, rst, org-mode, wikitext) and the language isn't stored then this choice makes text.