The requirements aren't clear. Please state for which app/model you wanna add an attachment. You could have a look at the chatter option that Odoo provides, here you can add that client as a follower on the record and then grant the client portal access from the Contacts app. Once you do it anything you add as an attachment in the chatter for that record will be seen by the client on the portal side.

If that doesn't solve the problem, please be a bit more specific about the requirement

Thanks and Regards,

Divyansh

@Divyansh thanks for your response. I want to upload a sample excel file to the customer portal so the customer can donwload it.