New .extra.tpl feature doesn't work with the numbered directory appended (for specific events, profiles or contribution pages)

I tried adding Main.extra.tpl to
/home/[[my-site]]/public_html/civicrm_custom_templates/CRM/Contribute/Form/Contribution/Main.extra.tpl
and there was no result, the file was not appended. It was a simple file that said just said "test".

/home/[[my-site]]/public_html/civicrm_custom_templates/CRM/Contribute/Form/Contribution/Main.tpl
this does seem to work just fine

this new feature was detailed by Greenberg here
http://civicrm.org/blogs/dave-greenberg/now-its-easier-add-custom-behaviors-templates