This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I've had a problem like this before. Steps that worked for me were:
1. Make that mime type addition and restart webserver 2. Clear file cache in the browser that downloaded the .jnlp file (it will still have a copy with the mine type set as text/html) 3. Try downloading again.