title: pre selected dropdown in angular material
body: <p>hi i using the angular material in angular 8 .</p>
  
  <p>i need to pre-selected <code>option</code> in dropdwown .</p>
  
  <p>i using this code :</p>
  
  <pre><code>&lt;mat-select *ngIf="justImage==true"&gt;
      &lt;mat-option selected (click)="setExtention('Picture',i)"&gt;
      {{ 'ENUM.FILE_TYPE.Picture' | translate }}
      &lt;/mat-option&gt;
  &lt;/mat-select&gt;
  </code></pre>
  
  <p>but it not selected in dropdown and disable that . it be disable but it not show selected items in dropdown .</p>
  
  <p>how can i solve this problem ????</p>