If JNLP files wont open with Java web launcher and you have tried the standard file association steps then try using a standalone app. 'Default Programs editor' https://defaultprogramseditor.com/ you do not need to elevate or install this application (its stand alone) and it needs to be run in the users profile with the problem.
1. Click File Type Settings 2. Click Context menu
3. Type jnlp, click Next 4. Click Edit selected Command
5. Click Browse Navigate to the current Java version 6. Click Save Context Menu, Click OK
C:\Program Files (x86)\Java\jre1.8.0_4*1\bin\javaws.exe
Click Next
7. Close the application
8. Test opening a JNLP file