When uploading attachments in Yahoo mail, there is a file called "uploader-2_7.swf" that is getting cached and not allowing the Attachment button to work as expected. By adding the following to your PWB INI file the file will not get cached and the button will work.
[Browser]
PreventSWFCaching=True
--Scott
Yahoo Attachments
Moderators: Tyler, Scott, PWB v2 Moderator