I used DragonUnPACKer trying to extract files from a game called Nosgoth, developed by Psyonix, the same authors from Rocket League. Unfortunately it seems these upk files are not supported by DragonUnPACKer. Nosgoth can still be downloaded for free through steam although it was cancelled, or if you want to have some sample upk file from this game let me know and I can send you.
I used DragonUnPACKer trying to extract files from a game called Nosgoth, developed by Psyonix, the same authors from Rocket League. Unfortunately it seems these upk files are not supported by DragonUnPACKer. Nosgoth can still be downloaded for free through steam although it was cancelled, or if you want to have some sample upk file from this game let me know and I can send you.