GroupWise does not allow deleting attachments from received email.
However, GroupWise 6.0 or above can recreate the message (using API method AddExistingMessage) minus attachments, and insert reference text, eg attachment caption, into the email message body.
Note that ‘recreate’ is different to cloning a message. Cloning is also possible in GroupWise, though it excludes part of the message (eg Properties), and is not available for received messages.
You can find examples that save selected messages and their attachments to disk here.