default_pageext in the setup file does not work for me.
I tried to set it as 'txt' and as a custom plugin I am developing but when I edit a page it only ever loads with Markdown selected.
Yes I am only trying to set it to loaded and working plugins.
ikiwiki version 3.20101129
default_pageextwith 3.20110124, and it works fine.
It seems to me from what you describe that you expect it to have an effect when you go and edit an existing page. That's not what it's for, it only chooses the default used when creating a new page.