The following seems to work:
Code:
{tags:'in_list($, ",", "kepub", ".kepub", "")'}
That will return ".kepub" if the tags contain "kepub". And nothing if it isn't
But, I have to ask, why? You can store kepubs in calibre. And if you send a book to a Kobo device using the KoboTouch driver, it can send either an epub or kepub. If you send a kepub, the driver names the file on the device as needed for it to be treated as a kepub. And if you have both formats for a book, you can set which the driver will send.