Congrats on the getting what sounds like a great deal on a very cool tractor.
What I've done to attach a spreadsheet is to edit the filename, replacing the suffix .xls with .txt. Then in TBN click "go advanced" then "manage attachments," browse to select and open the file, and click "upload" upload the file. Make sure to include the instruction "replace the .txt suffix with .xls to view this spreadsheet"
I attached a little spreadsheet as an example.
If you can't see the file extensions in Windows 7, click "Organize" then "folder and search options", then "view", then uncheck "Hide extensions for known file types"