def sub_type
      return nil unless @header['content-type']
      main, sub = *self['Content-Type'].split(';').first.to_s.split('/')
      return nil unless sub
      sub.strip
    end
def sub_type
      return nil unless @header['content-type']
      main, 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").