Is there an idiomatic file extension for Jinja templates?

I need to programatically distinguish between Jinja template files, other template files (such as ERB), and template-less plain text files.

According to Jinja documentation:

A Jinja template doesn’t need to have a specific extension: .html, .xml, or any other extension is just fine.

But what should I use when an explicit extension is required? .py is misleading, and any search including the words "jinja" and "extension" are badly searchwashed by discussion around Jinja Extensions.

I could easily dictate a project-wide convention (.jnj or .ja come to mind) but this is for open source so I don't want to buck the trend if there's already established practice somewhere.

EDIT 1: Again, I understand that the Jinja project -- purposefully -- does not define a default file extension. I'm asking if there are any unofficial conventions that have emerged for circumstances where one is desired for some project-specific reason.

EDIT 2: Clarification: This is not for HTML content.

Jinja Authors did not define a default extension. Most of Jinja template editors like Vim extension, TextMate extension, Emacs extension, and PyCharm mention no default extension to enforce Jinja highlighting.

Django had already a similar debate about setting a default extension, and ended as a wontfix issue. I quote from the closing message:

Filetype detection based on extension is flawed for the very reasons described in these comments, so you have to do some internal inspection, just like MIME type detection works.

I suggest that you should use your own since there is no common one.

Update: Things changed since I wrote this answer, .jinja2 and .j2 are trending.