[O] What Happened to org-export-html-extension?

[Ian Barton]

I am trying to publish a project where the files have a custom extension 
using the new exporter:

  ("org-ianbarton"
          :base-directory 
"~/Documents/emacs/web_sites/ianbarton/org/_posts"
          :base-extension "org"
          :publishing-directory 
"~/Documents/emacs/web_sites/ianbarton/pelican/blog/content"
          :recursive t
          :publishing-function org-html-publish-to-html
          :headline-levels 4             ; Just the default for this 
project.
          :auto-preamble t
          :auto-index f
          :html-extension "yml"
          :auto-preamble t
          :body-only t
    )

However, all files are published with an .html extension. My Emacs 
customize interface claims that org-export-html-extension doesn't exist. 
Grepping the source for org-export-html-extension doesn't find anything. 
So has the variable been renamed to something else?

Ian.