how to add new .webm mime types

The WebM file format isn’t known to WordPress by default, you have to add it.

add_filter( 'upload_mimes', 'custom_mimes' );
function custom_mimes( $mimes ){
    $mimes['webm'] = 'video/webm';
    return $mimes;
}

The .ogg file format is known to WordPress as audio/ogg, if you’re wanting to do video with it, the correct extension is .ogv.