Hello Philippe,
Data Type : User defined is meant to store any type of files you want. ZIP, MP3, DIVX, DOC, XLS, BlueRay, ... everything!
So you add the extension of the file, could it be zip, gzip, doc, iso, ...
as long as you have an application which can open the file with this extension on the workstation/browser, then there is no problem.
Best regards,
Alessandro