how to include view helpers in both action_controller and action_mailer templates (rails 3)

There's some terse documentation that helpers can be shared between
action_controller and action_mailer, but I have yet to find an example
of how this is done. Right now, I'm trying to use a "markdown" method
in the view templates for both. I have put the method in the
application helper (app/helpers/application_helper.rb). It works in
the view templates called from action_controller and descendents, but
not action_mailer. The action mailer templates return something like
this (from an rspec test):

  1) Request should have a send_reject_notice! method which sends a
rejection notice and saves
     Failure/Error: @request.send_reject_notice!
     undefined method `markdown' for #<#<Class:0x000000060441f0>:
     # ./app/views/request_mailer/reject_notice.html.erb:9:in
     # ./app/mailers/request_mailer.rb:8:in `reject_notice'
     # ./app/models/request.rb:81:in `send_reject_notice!'
     # ./spec/models/request_spec.rb:186:in `block (2 levels) in <top

Can someone explain what's going on?