Strange phenomenon here. I'm running Firefox and Gmail and for the last week or so, none of the attachments on incoming mail have been accessible. I have the paper clip icon, but it is in the upper right hand corner of the message (next to the new reply button) and clicking it has no effect.
I know the attachments are there, because I use Outlook as an email client and if I switch over to Outlook, the attachments can be opened! This problem occurs bot on my office PC (with a network) and the home PC (without).