You can attach files to an item (as Storm Machine said, if you have Premium), but only .doc or .pdf (I just learned that by trying it!) No Excel. Wuh-wuuuuuh.

Since you can't link your spreadsheet as a WDC item, if you want to link your work, you could upload the spreadsheet to OneDrive, make it public, and link it in your WDC item. But remember that you're not required to link your work, so you can also just say "stored in Excel" in your WDC item, or don't include an item number at all when you post that assignment.

Cheers,
Michelle