While performing inspections, you often need to reference other materials. Other source code files, Requirements and Design documents, etc. Currently Collaborator has no way to include these as...
In the meantime, I also just remembered that each file uploaded includes a "Notes" field which you could use to identify reference files - but I still like the idea of including a separate section for reference files because they would be easier to identify / group, plus would not count towards review file metrics.