```dart title="Dart" import 'package:kreuzberg/kreuzberg.dart'; Future main() async { // A Dart implementation of the `Validator` trait that gates on MIME type // cannot be plugged into the global validator registry. // `Kreuzberg.registerValidator(impl)` exists, but its // `createValidatorDartImpl` factory requires opaque `BoxFn*` closure // values whose constructors are not surfaced through // flutter_rust_bridge. // // Implement the validator in Rust and register it via `register_validator` // in a Rust shim crate. } ```