To fix the JNLP file not opening problem, update or install Java Runtime Environment, set JRE as the default app, or install JRE 64-bit.

Since JNLP files are simply Java Web Start files that open applications through an Internet browser, any browser that works in Windows 8.1 will open a JNLP file as long as you have Java installed.

net.sourceforge.jnlp.LaunchException: Fatal: Launch Error: Could not launch JNLP file. The application has not been initialized, for more information execute javaws/browser from the command line and ...

Like Java applets, your application will be constrained to a sandbox on the client unless you used signed jars and configure permissions correctly in your application's JNLP file.