You can upload documents to cases in two separate areas - via the Add Correspondence quick link or via the Correspondence section of the case.
Go to the Correspondence section of the case and click on Add Correspondence; or
Click on the Add Correspondence quick link in the header
Select the Type of correspondence: Case Document
Select a sub-type
Enter the attachment Title
Click on Select, find and select documents to upload
Choose either Save as Draft or Save + Complete to finish your upload