typescript mime type validator to a file picker in order pick png and jpg images

mime-type.validators.ts

import { AbstractControl } from '@angular/forms';
import { Observable, Observer } from 'rxjs';

export const mimeType =(
  control: AbstractControl
): Promise<{[key: string]:any}> | Observable<{[key: string]:any}> =>{
  const file =control.value as File;
  const fileReader = new FileReader();
  const frObs = Observable.create((observer : Observer<{[key: string]:any}>)=>{
    fileReader.addEventListener("loadend", ()=>{
        const arr = new Uint8Array(fileReader.result as ArrayBuffer).subarray(0,4);
        let header = "";
        let isValid = false;

    for(let i=0;i<arr.length; i++){
      header += arr[i].toString(16);
      }

    switch(header){
      case "89504e47": isValid=true; break;
      case "ffd8ffe1":
      case "ffd8ffe2":
      case "ffd8ffe3":
      case "ffd8ffe8": isValid=true; break;
      default:  isValid=false; break;
    }
    if(isValid){
      observer.next(null);
    }
    else{
      observer.next({invalidMimeType: true});
    }
    observer.complete();
    });
    fileReader.readAsArrayBuffer(file);
  });
  return frObs;
}