I've released an Alpha-Version of my project "Java-RPG-Maker-MV-Decrypter", which works with whole directories and save/renames files by itself...
It has a GUI, try it out! If you just want to decrypt single files, I recommend this version here. (Since you don't need to download it). For whole directories, you should try the Java version of the decrypter! =)
PNG-Files (Images - ".rpgmvp & .png_-Files") don't need the decryption code, except you want to re-encrypt them. Please go here for restoring images without a key.
In this area you can De- and (Re-)Encrypt Files from RPG-Maker MV & MZ Games.
You can get the decrypt code from the file:
Click "Detect" after you selected the System(.json) file. If you know the key, you can also just enter it in the text box.
In this area, you can restore PNG files (images - ".rpgmvp & .png_-Files") from RPG Maker games without the decryption key.