Resolution:
Custom rule to send a file:

ictl_mail_file(ToPoolName, MessageString) :-
    ricPool_getPoolNamed(ToPoolName, ToPoolObject),
    ictl_getCurrentTask(TaskObject),
    icrl_mail_file(TaskObject, ToPoolObject, MessageString),
    ictl_complete.             % complete fires the trigger, sending the mail
    
icrl_mail_file(TaskObject, PoolObject, MessageString) :-
    icrl_mailCreateEventSpec(TaskObject, EventSpecObject),
    icrl_mailCreateActionSpecFile(PoolObject, MessageString, ActionSpecObject),
    ricTrigger_create(EventSpecObject, ActionSpecObject, $IcRules Mail Trigger$, Trigger).

icrl_mailCreateActionSpecFile(PoolObject, MessageString, ActionSpecObjectOut) :-
    ic_enum(actionMailFormat, file, MailFormat),    % we are going to mail the message string
    ricActionSpec_createSendMail(PoolObject, MailFormat, MessageString, ActionSpecObjectOut).

%% generic version, passing in ActionType = {file, string, etc. }

ictl_mail_generic(ToPoolName, ActionType, MessageString) :-
    ricPool_getPoolNamed(ToPoolName, ToPoolObject),
    ictl_getCurrentTask(TaskObject),
    icrl_mail_generic(TaskObject, ToPoolObject, ActionType, MessageString),
    ictl_complete.             % complete fires the trigger, sending the mail

icrl_mail_generic(TaskObject, PoolObject, ActionType, MessageString) :-
    icrl_mailCreateEventSpec(TaskObject, EventSpecObject),
    icrl_mailCreateActionSpecGeneric(PoolObject, MessageString, ActionType, ActionSpecObject),
    ricTrigger_create(EventSpecObject, ActionSpecObject, $IcRules Mail Trigger$, Trigger).

icrl_mailCreateActionSpecGeneric(PoolObject, MessageString, ActionType, ActionSpecObjectOut) :-
    ic_enum(actionMailFormat, ActionType, MailFormat),    % we are going to mail the message string
    ricActionSpec_createSendMail(PoolObject, MailFormat, MessageString, ActionSpecObjectOut).

How do I send a file as an attachment using IcRule?