def text_transform(ftpp, filename, content):
    """
    If filename is *rss.xml*,
    replaces the string *__BLOG_ROOT__* by *self._root_web*.

    @param      ftpp        object FolderTransferFTP
    @param      filename    filename
    @param      content     content of the file
    @return                 new content
    """
    if filename.endswith('rss.xml'):
        web = ftpp._root_web
        if not web.startswith('http://'):
            web = 'http://' + web.strip('/')
        ftpp.fLOG('[text_transform] replace __BLOG_ROOT__ by ', web)
        return content.replace('__BLOG_ROOT__', web)
    else:
        return content
