def sub_type
    return nil unless @header['content-type']
    _, sub = *self['Content-Type'].split(';').first.to_s.split('/')
    return nil unless sub
    sub.strip
  end
Returns a content type string such as "html".
This method returns nil if Content-Type: header field does not exist
or sub-type is not given (e.g. "Content-Type: text").