Unless I am missing something, nothing appear to happen for the shortcut file when using the ItemAttachmentAdd Token. Works fine for the actual file. Also noticed, the FileExists(“C:tempShortcut test.pdf.lnk”) method return FALSE for the shortcut item. Do you see the same behaviour?
Regardless of the different behaviour, I think you can not skip the pop-up dialog of the attach file/attach shortcut. Another option may be to use the Object API to create the message, attach the file and finally open the message for the user using the ItemOpen() token.
Hope this helps.