when approver of any RITM opens the RITM the attachment shouldnt be visible to them .

[NiloferS]

the attachment of the RITM shouldnt be visible to the approvers of that particular item. This scenario is not restricted to one catalog item. Its applicable for all. 

Kindly guide me. 

[Mark Manders]

Is this just for the approvers? Because if they have sufficient rights to see the RITM, they also have the right to see the attachment. 

If, for instance, manager X needs to approve a request for employee Y and X has ITIL, he can see the attachment. In fact: if he has a role within fulfillment, you don't want this limitation. 

You could make ACL's or a Query Business Rule on the sys_attachment table so only the assigned to or members of the assignment group and the opened_by/requested_for can see them, but check if this will always work withing your organization. 

If you are planning on upgrading to Xanadu soon: the deny-unless ACL's will make this a very easy setup.