I've had a few problems with outlook refusing to open attachments the error is:
Your organisations polices are preventing us from completing this action for you, for more info please contact your helpdesk.
It's caused by a recent install of firefox
You need to edit the registry.
You need to edit the (Default) from FirefoxHTML to htmlfile
You will also need to do this for .htm .xhtm .xhtml .shtml .xht and possibly .xhtmm but I've not seen that one on the systems I've seen.