Start xwiki in tomcat, check /usr/share/tomcat5.5/temp (or whatever is the result of calling System.getProperty("java.io.tmpdir"))
Upload a file,
note the new temp file
note the file is still there.
commons.fileupload gives files names which are guaranteed unique for that classloader. When the servlet is restarted, the filenames are no longer guaranteed unique and this might cause subtle and difficult bugs.