Downloading Nyquist plug-ins

There has been a post to our feedback email address describing a problem when downloading Nyquist plug-ins.

The problem as described appears to be that the file name is modified automatically on download by “.txt” being appended to the file name. Of course, if this happens, the plug-in will not be recognised by Audacity.

Is this a common problem on macOS?
Why does it happen?
What are the solutions?
Do we say anything about this in the manual?

Just tried it on Mavericks with BassToCenter.ny. No .txt added…

But I’m using RCdefault app 2.1 (an old preference pane), by Carl Lindberg (RubiCode) to map extensions. Don’t think that has any influence, but I don’t know for sure. The main use of this pref pane is to make sure .txt and a number of other extensions open in TextWrangler, so Text Editor doesn’t screw them up. See: