Untitled

  alternative = Mail.new do
    text_part do
      content_type 'text/plain; charset=UTF-8'
      body 'Plain text content.'
    end

    html_part do
      content_type 'text/html; charset=UTF-8'
      body '<html><b>HTML</b> content.</html>'
    end
  end

  mixed = Mail.new do
    to 'YOU <you@foo.com>'
    from 'ME <me@foo.com>'
    subject 'Trick into multipart/mixed.'

    body_part = Mail::Part.new
    body_part.body = alternative.body.encoded
    body_part.content_type = alternative.content_type
    add_part(body_part)

    add_file(:filename => 'file.txt', :content => 'FILE CONTENTS HERE')
  end

  mail.deliver