OPEN:
/components/com_k2/templates/default/item.php

FIND LINE that starts with:
<a title="<?php echo htmlentities($attachment->titleAttribute, ENT_QUOTES, 'UTF-8'); ?>"

REPLACE all line with:
<a title="<?php echo htmlentities($attachment->titleAttribute, ENT_QUOTES, 'UTF-8'); ?>" href="<?php echo JRoute::_('media'.DS.'k2'.DS.'attachments'.DS.$attachment->filename); ?>">