Filter / Let attachments be approved if they don't have an extension

Hi,

we have rules to filter specific extensions to be approved by admins, before they are delivered. 

That works as expected. 

But how do I filter files, that do not have any extensions (or deny them). 
Lately those appear to be delivered, but they are usually never good attachments, so I want to block them or have them approved. 

I didn't find any solution, as I can say *.zip, but this is just like

"fileabcde" instead of "fileabcde.zip". 

Probably you can help. 

Thanks

Patrick