I know that new code belongs in lib/private/Preview/*, and I’ve managed to cobble together code for several new mimetypes. Placing the code there (and enabling it in config.php) doesn’t activate this code.
I’ve installed composer, and I attempted to do “composer update”. This added references to my new code in autoload files (2 of them I think). But still the code is inert, even making big syntax errors in it doesn’t cause problems (while making even a minor one in MP3.php causes things to explode).
Obviously I’m missing some part of the build process. I have no idea what that might be, and nothing in the build/* directory is obvious. Can someone clue me in?