Laravel Nova Images Field callback Function outside

in the nova resource I can define an image field with a media library addon package like:

Images::make('Main image', 'media_main')
    ->setFileName(static function($originalFilename, $extension) {
         return md5($originalFilename) . '.' . $extension;
}),

How can I use a function from the resource so that I can use the function multiple times for more Images fields and don’t use duplicate code always for the same functionality.

For example:

public static function fileName($originalFilename, $extension) {
    return md5($originalFilename) . '.' . $extension;
}

So that in the fields section I can use multiple times:

Images::make('Main image', 'media_main')
    ->setFileName(self::fileName($originalFilename, $extension)),

The last call always results in an error message, because I can not grab the parameters. What I’m doing wrong? I think I have a misconception …