Hi. I just found this thread by searching how to edit this file type in notepad++ as I want to edit GTA5 file. Anyway I did not found the way to open/edit this file in Notepad++ but it can be done in "OpenIV". I wanted to make it easier to edit in Notepad++ but I did not found decrypting solution.
Anyway I may need to do some modding for RDR2 in the future. As of now I dont own this game sadly.
So again use "OpenIV" to edit this file type. Good luck.