You have to have the message open (not just previewing) in the new mail folder and then close it to confirm the deletion.

If I delete an attachement to an email (in the New Mail or any other folder), the attachment appears to be deleted, leaving a residual tag with minimal size indicating a deleted attachment.  But on simply switching to the message screen and back to the attachment, it reappears.  Closing and restarting PMAIL and ditto the whole computer makes no difference.  Am I missing something ?