This is a two-part look-up:
First, you look up the default value of HKEY_CLASSES_ROOT\[file_extension]. For your extensions, .jnlp, the value is „JNLPFile“. Let’s call this the [file_descriptor].
Now you can look up the default value of HKEY_CLASSES_ROOT\[file_descriptor]\Shell\[action]\command (where [action] is the shell action you are interested in, e.g.: Open, Print, Edit, etc.).
e.g. „.txt“ à „txtfile“
Stand: 2023-02-10