Something like this should create a (non-empty) FileDocument containing your XLS:
import java.io.FileInputStream;
import system.proxies.FileDocument;
import com.mendix.core.Core;
final FileDocument fileDocument = FileDocument.create(this.getContext());
Core.storeFileDocumentContent(this.getContext(), fileDocument.getMendixObject(),
"Output.xls", new FileInputStream("C:/Users/Jeroen/Documents/Output.xls"));