This problem is because SAP does not checks for all published application objects whether there are attachments or not.
In order to make the SAP check this.
In TC->SM 30 use the table SXPARAMS.
Create new Parameter: GOS_ATTA_CHECK_ALL with Value: X
Before the attaching a file that Attachment List is empty for Purchase Order “4500017510”, as there is either are no above parameter placed in the SXPARAMS or no documents yet attached .
Through TC->SM30 for table SXPARAMS, Parameter GOS_ATTA_CHECK_ALL is maintained with Value: X
Document attached through OAOR
Now the attachment shows up through Attachment list
Similar way they should show up all attachments in the Attachment list for other Purchase documents previously Attached.