validate method
Performs type coercion against an input
value, and returns a list of errors if the validation was unsuccessful.
Implementation
@override
ValidationResult<UploadedFile> validate(String key, UploadedFile input) {
//if (input is! UploadedFile) {
// return _Vr(false, errors: ['Expected "$key" to be a boolean.']);
//}
return _Vr(true, value: input, errors: []);
}