form:FormGroup;
  constructor(private fb:FormBuilder) { }
 
  initForm() {
    this.form = this.fb.group({
      'files':['', [this.fv.bind(this)]]
    });
  }
 
  /**
   * my validator
   */
  fv(fc:AbstractControl) {
    console.log('validate', fc);
    // ...
  }
 
  /**
   * func that need to revalidate 'files' field
   */
  someFunc() {
    // ...
    this.form.updateValueAndValidity(); // NOT WORKING, fv validator not called
    this.form.get('files').updateValueAndValidity(); // WORKING!, fv validator CALLED
   }