application/x-java-jnlp-file is not a supported format. You cannot mix text/html and application/x-java-jnlp-file in the same response
Thanks for the reply!
Solution to the problem:
Have the portlet generate a string containing the jnlp content dynamically and write this out to a file at a certain location. Then, have a link inside the portlet pointing to the file - the user will click on it and the java web start sequence will initiate.