I think this will help
<directoryBrowse enabled="false" />
<defaultDocument>
<files>
<clear />
<add value="index.aspx" />
<add value="Default.htm" />
<add value="Default.asp" />
<add value="index.htm" />
<add value="index.html" />
<add value="iisstart.htm" />
<add value="default.aspx" />
<add value="index.php" />
</files>
</defaultDocument>
</system.webServer>