mt.rb

require "mail"

mail = Mail.new do
  to      '[email protected]'
  from    'Mikel Lindsaar <[email protected]>'
  subject 'First multipart email sent with Mail'
end

text = 'Find my source code attached'
html = 'Find my <b>source code</b> attached'

html_part = Mail::Part.new do
  content_type  'text/html; charset=UTF-8'
  body          html
end

text_part = Mail::Part.new do
  body          text
end

mail.part :content_type => "multipart/alternative" do |p|
  p.html_part = html_part
  p.text_part = text_part
end

mail.attachments['some.rb'] = { content: File.read(__FILE__), mime_type: "text/ruby" }

# Don't know if this is really necessary:
#
# mail.content_type = mail.content_type.gsub('alternative', 'mixed')
# mail.charset= 'UTF-8'
# mail.content_transfer_encoding = 'quoted-printable'

puts mail.to_s