This can have two causes:
1. You may not upload files with the (obsolete) extension .XLS.
Remedy: Open the file with Excel and then save it as .XLSX.
2. Your documents folder is set in such a way that you cannot add Excel documents.
In that case:
- Go to the appropriate folder
- Open the folder properties
- Click on the "Document Types and Templates" tab
- Verify that the document type "Excel" is in the list
- If not, change this setting to allow this document type