Angular – Validator in an input file field

In angular, how can I make a obrigatory field validator for a field that will be inserted a file in? Validator.required is not working in this case.


<input class="input" type="text" formControlName="document" value="{{arquivosSelecionados[0] ? arquivosSelecionados[0][0].name : ''}}" disabled>
<app-form-messages class="md-inputfield"  [field]="formIncluirMembros.get('document')"></app-form-messages>


this.formIncluirMembros.addControl("document", new FormControl(null, Validators.required));

Source: Angular Questions