Sweet thanks. I was trying to use the new analytics and it doesn't seem this option is in there.

For whatever reason, you can't join transactions to files in an analytics workbook, or the reverse if you started with a file record

I ran into the same problem myself, and was quite disappointed in that limitation. I even tried creating a formula field just to return a File id, but couldn't see that field in the workbook. They really don't want to let you join transactions and files for some reason.

I firmly believe that Analytics is still in "Beta" and that it will be a perpetual work in progress.