Allow exclusion of certain MIME types from validation

This looks like a great feature and a nice implementation - thank you. Because we're adding new config here, I think we'll need to add it to the default values in config/install/mimedetect.settings.yml and perhaps also add an update hook to set it as a new default value for sites that already have the module installed?

Also if we can get https://www.drupal.org/project/mimedetect/issues/3160994 → committed before this one, then we'll need to add the new config value to the schema here, too.