Multipart Alternative + Multipart Related Email with ActionMailer for Rails 3

embedded_mailer.rb

class EmbeddedMailer < ActionMailer::Base

  def embedded_mail(to, from, subject, html_message, text_message, files = {}, embeds = {})
    mail(:to => to, :from => from, :subject => subject) do |format|
      format.text { render :text => text_message }
      format.html { render :text => html_message }
    end
  
    # Embeds
    embeds.each do |key, embed|
      attachments.inline[key] = {
        :content => File.read(embed[:path]),
        :content_id => "<#{key}>"
      }
    end

    # Attachments
    files.each do |key, embed|
      attachments[key] = File.read(embed[:path])
    end
  end

end

Blog Post: http://www.shortround.net/2011/04/07/multipart-alternative-multipart-related-email-with-actionmailer-for-rails-3/