Local JNLP file not read for jenkins slave
Answer by @gareth_bowles is wrong at least today, the url will work as a file url as well. Don't know if this was changed at some point.
For our windows slave it is "file:/D:/jenkins-slave/slave-agent.jnlp", which works.
The jnlpUrl parameter needs to be an http or https link to your Jenkins master, e.g. http://jenkins.yourco.com/computer/slavename/slave-agent.jnlp -it won't work as a file URL.