class ActionMailer::Base

def wrap_inline_attachments(message)
  # If we have both types of attachment, wrap all the inline attachments
  # in multipart/related, but not the actual attachments
  if message.attachments.detect(&:inline?) && message.attachments.detect { |a| !a.inline? }
    related = Mail::Part.new
    related.content_type = "multipart/related"
    mixed = [ related ]
    message.parts.each do |p|
      if p.attachment? && !p.inline?
        mixed << p
      else
        related.add_part(p)
      end
    end
    message.parts.clear
    mixed.each { |c| message.add_part(c) }
  end
end