I was trying to serve a JavaScript file with Python's http.server module, but I was getting this error:

Loading module from “http://localhost:9876/main.js” was blocked because of a disallowed MIME type (“text/plain”).

I saw in this StackOverflow answer that I needed to change the MIME type for JavaScript files. I'm using Windows, so I checked the key HKEY_CLASSES_ROOT\.js in RegEdit, and I found that it had this value:

(Default)     JSFile
ContentType   text/plain
PerceivedType text

I changed the ContentType to application/javascript, and now my JavaScript files are served with the correct MIME type.

I'm not sure why the MIME type was set to text/plain in the first place. I'm guessing that it was set by a program that I installed, but I don't know which one. I'm also not sure if changing the MIME type will cause any problems with other programs.