[mkl]

There's no option for that when submitting. I think it's probably moderator-applied.

[Tomte]

It usually happens automatically, but obviously not this time. That makes it very hard to think about it, because it mostly works.

My hypothesis is that HN's detection only looks for file extension, not content type headers, because it doesn't fetch the submission itself.

[scott_s]

grzm is correct, it does not happen automatically. See https://news.ycombinator.com/newsguidelines.html.

[grzm]

It's just text as part of the title. The submitter can do it (until the edit time runs out).