message: Fixes switch case in course media html
diff: <mat-icon *ngSwitchCase="'image'">image</mat-icon>
  <mat-icon *ngSwitchCase="'pdf'">pdf</mat-icon>
  <mat-icon *ngSwitchCase="'zip'">zip</mat-icon>
  - <mat-icon *ngSwitchCase="'unknown'" *ngSwitchDefault>file</mat-icon>
  + <mat-icon *ngSwitchDefault>file</mat-icon>
  </ng-container>
  </div>
  