message: App: Add BackendService methods getMimeType & head
diff: @@ -22,6 +22,17 @@ export class BackendService {
  return throwError(err);
  }
  + head(serviceUrl: string): Observable<any> {
  + return this.http.head(BackendService.API_URL + serviceUrl, {headers: this.authenticationService.authHeader(),
  + observe: 'response', responseType: 'blob'})
  + .pipe(catchError(this.handleUnauthorized));
  + }
  +
  + async getMimeType(serviceUrl: string): Promise<string> {
  + const response = await this.head(serviceUrl).toPromise();
  + return response.body.type;
  + }
  +
  get(serviceUrl: string): Observable<any> {
  return this.http.get(BackendService.API_URL + serviceUrl, {headers: this.authenticationService.authHeader()})
  .pipe(catchError(this.handleUnauthorized));
  