title: How to use required validation on <input type="file"> in angular 4
body: <p>Text input are working properly. I used same for file but its not working.</p>
  
  <pre><code>&lt;input type="text" name="title" #title="ngModel" ngModel required class="form-control" placeholder="Title Here"&gt;
  &lt;div *ngIf="title.errors &amp;&amp; (title.dirty || title.touched)"&gt;
      &lt;span class="help-block error" [hidden]="!title.errors.required"&gt; Title 
        is Required 
      &lt;/span&gt;
  &lt;/div&gt;
  &lt;input type="file" #image="ngModel" (change)="fileEvent($event)" 
  name="image" ngModel&gt;
  </code></pre>