API or Deluge script for Zoho Books Attachment Custom Field

API or Deluge script for Zoho Books Attachment Custom Field

I have created a Custom Field with field type as Attachment (Not the default Attachments) for Contact module in Zoho Books. This field should be populated whenever a record is created or the Image (Attachment field) shall be updated from external system either Zoho CRM or Php application.

The documentation for Zoho Books API does not mention on how to upload files for Custom Field of Attachment Field type, there documentation has references for the attachments in module.
Can you point some references to upload files to Custom Field of Attachment field type ? This is an important requirement to move forward and we are stuck.

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