Error on Server rules

Hi,I tried to put a rule on the message settings document on the server, which would not deliver emails with suspicious attachments like .SCR, .COM, .EXE etc.

but this rule doesnt seem to work as after i put in the conditions and try to save it it gives an error message

“Error on Server Rules - Sub Queryopendocument → 4412: Notes Error - cannot allocate field”

Any Ideas anyone how to resolve this.

Cheers

Avi