Using SSO Authentication to call Siebel Rest API for attachment

Our current version is Siebel 21.1. We're trying to get attachments from Siebel without applets and had no success in calling EAI Siebel Adapter directly with business services, since Siebel business services can't handle the base64 string it returns for the attachment. We were suggested the transcode business service, but that chops off the file when it converts so PDF/anything besides a simple txt file. That makes the file unable to be opened once downloaded. We're pretty much stuck on that part so now we're trying to call it via web service with OpenUI (technically just javascript) which works with