Deleting attachments when message signed with S/MIME

TBird 1.5, public release, WinXP.

When an e-mail has been either signed or encrypted with an 
S/MIME signature, the delete-attachment function is not 
available.  Those options are all grayed out on the menus. 
When the message has been neither signed nor encrypted, 
attachment deletion functions correctly.

Any suggestions for making this work will be greatly 
appreciated.  Thank you!

-- 
RJW