Take a look at the AdminCMSPortlet. The upload.jsp has something like this:

<form name="uploadform" method="post" enctype="multipart/form-data" action= ...

Make sure you are using the most recent version of fileupload that allows for portlet fileuploads. Previous versions did not.

The code you're looking for begins with this:

DiskFileItemFactory factory = new DiskFileItemFactory();
 PortletFileUpload upload = new PortletFileUpload(factory); // TODO: configure uploader
 String sPath = "";
 List fileItems = upload.parseRequest(aReq); // TODO: mod interface to allow multiple uploads.
 Iterator itr = fileItems.iterator();
 while(itr.hasNext())
 {
 FileItem item = (FileItem) itr.next();