Thank you for your posting. I can confirm that I can also duplicate this failure with the 6.5.4 beta GroupWise client. It would appear Novell have broken the underlying ItemAttachmentGetCurrentIndex() token used by this language extension.
I have written to Novell this morning to ascertain the status of this command. I’ll report my finding via this thread.
At this stage I cannot offer any workarounds that would let you identify the selected attachment.