class Mail::Message

def add_part(part)
  if !body.multipart? && !Utilities.blank?(self.body.decoded)
    @text_part = Mail::Part.new('Content-Type: text/plain;')
    @text_part.body = body.decoded
    self.body << @text_part
    add_multipart_alternate_header
  end
  add_boundary
  self.body << part
end