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):

Failures:   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>: 0x00000006041d88>      # ./app/views/request_mailer/reject_notice.html.erb:9:in `_app_views_request_mailer_reject_notice_html_erb__435172659881060762_50505700__363986106511591665'      # ./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 (required)>'

Can someone explain what's going on?