docs: def add_content(self, content, mime_type=None):
          """Add content to the email
  
          :param contents: Content to be added to the email
          :type contents: Content
          :param mime_type: Override the mime type
          :type mime_type: MimeType, str
          """
          if isinstance(content, str):
              content = Content(mime_type, content)
          # Content of mime type text/plain must always come first
          if content.mime_type == "text/plain":
              self._contents = self._ensure_insert(content, self._contents)
          else:
              if self._contents:
                  index = len(self._contents)
              else:
                  index = 0
              self._contents = self._ensure_append(
                  content, self._contents, index=index)