hi @Ankur Bawiskar 

thanks for your response.

and i also don't want to allow user to add new attachments in the specific catalog item. 

i started with before BR to implement this but in one of article i read this ...

https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0825743

in additional info they have mentioned this >>

This business rule works primarily to prevent deletion of the record in the sys_attachment table.  If when reloading the task, the attachment reappears with an attachment of 0 bytes; this is because the base64 data for the file was deleted from the sys_attachment_doc table which resulted in a broken attachment link with an empty file.

Solve this by adding a matching "on before delete" business rule to both of these tables:
sys_attachment
sys_attachment_doc