Editing the contents of
works just fine on Sublime Text 3.0 (this is my install directory on Ubuntu 18, yours might be different)
I modified this line to include |"text/paperscript"
Note: this file is inside a zip archive and you should see a prompt asking if you wish to update the contents in the file, obviously, click update
One last thing. By default, only the root user has access to the directory and it's contents, so you might want to change ownership recursively or update write permission for other users. See link below for reference