Problem downloading plugins

With OS Catalina when I download a plugin from “” I get a text file. I did this years ago but now I cannot seem to get the hang of it. I would appreciate any direction offered. Thank you.

“Nyquist plug-in” are just text files, but with a file extension (the part of the file name after the dot) of “.ny” rather than “.txt”.
Unfortunately, Safari has a habit of adding “.txt” to the end of the file name.
Fortunately the “Nyquist Plug-in Installer” can fix that automatically.