DTML Method, index_html::
<dtml-call "REQUEST.set('default', getDefaultFile)">
<dtml-var expr="_.getitem(_['default'], 1)">

(Script) Python, getDefaultFile::
defaultFiles=['index.htm', 'index.html', 'index.php', 'index.php3', 'default.asp', 'home.jsp']
for default in defaultFiles:
    if hasattr(context, default): return getattr(context, default).getId()