BIP Webservice returns a CID value - no data

Hi

Customer is making a call (SOAPUI) to the BI Publisher webservice - PublicReportService.runReport() - to execute my report and return the data, normally works fine, except I have one big report that where it does not put the data in the response payload, it just sends a short reference back (e.g. <reportBytes>cid1262646072216</reportBytes>). This seems to be a "content ID" value, presumably to an attachment or file somewhere. Problem is it is not available anywhere, and cannot find what or where this is supposed to be.

I searched docs, guides, KM, bugs, google, and nothing on this!