(assuming you're using some variant of UNIX/Linux...)
I had a problem with attachments not working, and it had to do with the upload directory not having the proper permissions.
There should be an 'upload' dir in your installation. Make sure that it is writable by the user that is running your
servlet engine. In my case the directory was created by a regular user, and was writable only by the owner (744 or something like that).
Tomcat was running as a 'tomcat' user and could not write to the directory.
[originally posted on jforum.net by heehaw]