class Mail::Message

def html_part=(msg)

message and set itself that way.
text_part are both defined in a message, then it will be a multipart/alternative
Helper to add a html part to a multipart/alternative email. If this and 
def html_part=(msg)
  # Assign the html part and set multipart/alternative if there's a text part.
  if msg
    msg = Mail::Part.new(:body => msg) unless msg.kind_of?(Mail::Message)
    @html_part = msg
    @html_part.content_type = 'text/html' unless @html_part.has_content_type?
    add_multipart_alternate_header if text_part
    add_part @html_part
  # If nil, delete the html part and back out of multipart/alternative.
  elsif @html_part
    parts.delete_if { |p| p.object_id == @html_part.object_id }
    @html_part = nil
    if text_part
      self.content_type = nil
      body.boundary = nil
    end
  end
end