Renderer: use Convertible's way of picking the last Converter's output extension

lib/jekyll/renderer.rb

@@ -25,10 +25,20 @@ module Jekyll
       @output_ext ||= if document.permalink
         File.extname(document.permalink)
       else
-        converters.first.output_ext(document.extname)
+        if output_exts.size == 1
+          output_exts.last
+        else
+          output_exts[-2]
+        end
       end
     end
 
+    def output_exts
+      @output_exts ||= converters.map do |c|
+        c.output_ext(document.extname)
+      end.compact
+    end
+
     ######################
     ## DAT RENDER THO
     ######################