PO Attachment not working through ErpObjectAttachmentService

Summary:

Requirement is to upload the attachment to Open POs but PO Attachment is not working when we are trying to do through Soap UI - fscmService/ErpObjectAttachmentService and using entity name as PO_HEADERS. It keep showing attachment Failed.

But the same API - ErpObjectAttachmentService is working for Purchase Agreement.

Any help will be appreciated here on how to upload attachment on Open POs

Note:-

Purchase Agreement are getting created through FBDI and Import Purchase Agreement Program

Purchase Order are getting created through FBDI and Import Orders Program