Instead of giving up on custom mime types without blocks, Rails should attempt to render the standard template. Currently you get this useless exception, "Expected a block but none was given for custom mime handler #{mime_type}"
I added a simple patch that tries to render the custom format's template.
respond_to do |format| format.mobile # action.mobile.erb format.opml # action.opml.builder end
Ticket #8174 (http://dev.rubyonrails.org/ticket/8174)
