add the content of the file(s) (or the text in string) in HTML body

def add_content(self, text=None, *markdown_files):
        """add the content of the file(s) (or the text in string) in HTML body"""
        for markdown_file in markdown_files:
            self.markdown_text += self._text_file(markdown_file)

        if text:
            self.markdown_text += text