Seems like a techy question for the new tech board!

Questions go on the questions board.

uhhh, fug... how do you do it again..

$config['allowed_ext_files'][] = 'webm';

...........

// WebM Settings
$config['webm']['use_ffmpeg'] = true;
$config['webm']['expected_format'] = array('webm' => 'webm', 'mp4' => 'mp4');
$config['webm']['video_codecs'] = array('vp8', 'vp9', 'h264', 'av1');
$config['webm']['allow_audio'] = true;
$config['webm']['max_length'] = 90000;
$config['webm']['ffmpeg_path'] = 'ffmpeg';
$config['webm']['ffprobe_path'] = 'ffprobe';

Have ffmpeg , ffprobe installed on your server device

also you probably need google's webm package that likely is installed by apt install webm-dev

how to disallow webp?

>>381
by not having it in the allowed_ext

It's quite well documented in the config.php file (that you're not supposed to edit, use instance-config.php to overwrite default valuse). There's comments everywhere and what it does. Just mess around with it.
Why disable mp4 and allow webm? mp4 has highest support.
I thank you greatly for using vichan. It's far less of an eyesore as a user than all the newer ones.
Wakaba is pretty decent too in terms of being a pleasant user experience, but it's a lot more hassle for the admin.