[Zope] dtml-tree '+' in discussion reply list causes 'Download' window prompt

Hi, Zope CMF showThreads creates a list of replies on a item and a reply has an expandable '+' to view replies it has of its own. When someone clicks on the '+' to expand the tree a prompt box is displayed saying 'you want to download this document'. I know this is more for the CMF list but I have had no ideas. Thought maybe this was something I am doing wrong with the dtml-tree tag. Have I missed a permission or something? Thanks for your help. Richard