progress, I added a script action (below) to the topic right after case creation and now the attachment is on the sys_attachment table. 

(function execute() {

    vaSystem.attachToRecord(vaInputs.file_picker.getValue(), 'sn_hr_core_case', vaInputs.create_sn_hr_core_case);

})()

I've tested creating two hr cases and see the attachments on two different tables, the first one is on sys_cs_conversation_task and the second one is on sys_cs_conversation. I'm not sure what to change in the GlideSysAttachment.copy script

This is what I tried

GlideSysAttachment.copy('sys_cs_conversation_task', vaVars._attachment_record_sysid, 'sn_hr_core_case', gr.sys_id);