org-html-publish-to-html is defined in ox-html.el and looks like this (sans doc):(defun org-html-publish-to-html (plist filename pub-dir) (org-publish-org-to 'html filename ".html" plist pub-dir)) You could define your own publishing function, e.g. (defun my-org-html-publish-to-html (plist filename pub-dir) (org-publish-org-to 'html filename ".yml" plist pub-dir)) and refer to that in your org-publish-project-alist. org-html-extension is used in org-html-export-to-html which calls org-export-to-file, which is the same function org-publish-org-to calls (so org-html-extension is bypassed). There's also the export option :html-extension but it doesn't look like it's used to name published files.
Thanks for the clear explanation. Using my own function works, but this does seem to be a regression. Defining your own extension is useful e.g. if you want to emit .php files.
Ian.
