title: Angular2: validation for <input type="file"/> won't trigger when changing the file to upload
body: <p>Angular 2 seems to have troubles with running validation when a file input changes. </p>
  
  <p>I made a plunk to illustrate this problem: </p>
  
  <p>I make a formGroup like</p>
  
  <pre><code>this.frm = new FormGroup({
      file: new FormControl("", this.validateFile)
  });
  </code></pre>
  
  <p>And in the validateFile function I throw an alert and log to the console:</p>
  
  <pre><code>public validateFile(formControl: FormControl): {[key: string]: any; } {
     alert('Validation ran');
     console.log('Validation ran');
  }
  </code></pre>
  
  <p>Plunkr to illustrate the issue: <a href="https://plnkr.co/edit/Pgcg4IkejgaH5YgbY3Ar?p=preview" rel="noreferrer">https://plnkr.co/edit/Pgcg4IkejgaH5YgbY3Ar?p=preview</a></p>
  
  <p>The validation will run when initializing the page but won't run each time you change the file to be uploaded.</p>
  
  <p>Is there any solution to this problem?</p>