There are schemas for various types of documents (Excel, Word, etc.) There are also various schemas for the versions as well.
This example creates an XLS file using the Excel 2003 schema. When run, it will create a file in c:\tmp\msofficedocuments (the directory will be created automatically) that you can open normally by double-clicking the file (if you have Excel 2003 installed.)
More information can be found at Microsoft's Download Center. (A little bit of tweaking has to be done because the schemas include other schemas in the distribution.) This isn't really a "TIBCO thing" since you could (obviously) use this schema from anywhere, but as I've had uses to create Excel files from TIBCO I thought I'd throw the example out there as something you could do to create something a little fancier than CSV files.
Let me know if you have any questions. This was done using Designer 5.6.