Multipart Alternative + Multipart Related Email with ActionMailer for Rails 2

embedded_mailer.rb

class EmbeddedMailer < ActionMailer::Base

  def embedded_mail(to, from, subject, html_message, text_message, files = {}, embeds = {})
    # Hack TMail to allow embeds
    TMail::HeaderField::FNAME_TO_CLASS.delete 'content-id'

    recipients   to
    from         from
    subject      subject

    content_type 'multipart/mixed'

    # The Message
    part :content_type => 'multipart/alternative' do |m|

      # Plain Text Message
      m.part :content_type => 'text/plain' do |p|
        p.transfer_encoding = 'base64'
        p.body = text_message
      end

      # HTML Message
      m.part :content_type => 'multipart/related' do |x|
        x.part :content_type => 'text/html' do |p|
          p.body = html_message
        end

        # Embedded
        embeds.each do |key, embed|
          x.part :content_type => embed[:content_type] do |p|
            p.headers = { 'Content-ID' => "<#{key}>" }
            p.transfer_encoding = 'base64'
            p.body = File.read embed[:path]
          end
        end
      end
    end
  
    # Attachments
    files.each do |key, embed|
      attachment :content_type => embed[:content_type] do |a|
        a.filename = key
        a.body = File.read embed[:path]
      end
    end
  end

end

Blog Post: http://www.shortround.net/2010/11/24/multipart-alternative-multipart-related-email-with-actionmailer/