[ACA-1695] viewer extensions (#576)

[ACA-1695] viewer extensions

src/app/components/preview/preview.module.ts

@@ -35,6 +35,7 @@ import { PreviewComponent } from './preview.component';
 import { ViewUtilService } from './view-util.service';
 
 import * as pdfjsLib from 'pdfjs-dist';
+import { PreviewExtensionComponent } from './preview-extension.component';
 pdfjsLib.PDFJS.workerSrc = 'pdf.worker.js';
 pdfjsLib.PDFJS.disableFontFace = true;
 
@@ -57,6 +58,7 @@ const routes: Routes = [
     ],
     declarations: [
         PreviewComponent,
+        PreviewExtensionComponent
     ],
     providers: [
         ViewUtilService