class Mail::Message

def text_part=(msg)

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