You're looking at a .twbx (packaged workbook) file, not .twb. Unzip the file and you will get the .twb from it, along with a folder that contains the data for the package.
Great, thanks Toby.
Is there an automatic way to find out if a workbook on the server is a .twb or .twbx ? In other words, is this info stored in the postgreSQL database?