Upload fails when xwiki.upload.tempdir is not set

The parameter xwiki.upload.tempdir is used to specify where to put files larger than the in-memory threshold. On some (linux) systems, if this path is not set, the upload action fails, throwing:

java.lang.NullPointerException
at com.xpn.xwiki.web.UploadAction.action(UploadAction.java:51)

This should be fixed by checking if the default location is writable, and providing a default writable path, like /tmp, or TOMCAT_HOME/temp on linux.