mean-guide-frontend/src/app/posts/post-create/mime-type.validator.ts

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

export const mimeType = (
  control: AbstractControl
): Promise<{ [key: string]: any }> | Observable<{ [key: string]: any }> => {
  console.log(control.value)
  if (typeof control.value === 'string'){
    return of(null);
  }
  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);
        }
        console.log(`header: ${header}`);
        switch (header) {
          case '89504e47':
            isValid = true;
            break;
          case 'ffd8ffe0':
          case 'ffd8ffe1':
          case 'ffd8ffe2':
          case 'ffd8ffe3':
          case 'ffd8ffe8':
          case 'ffd8ffdb':
            isValid = true;
            break;
          default:
            isValid = false; // Or you can use the blob.type as fallback
            break;
        }
        if (isValid) {
          observer.next(null);
        } else {
          observer.next({
            invalidMimeType: true,
          });
        }
        observer.complete();
      });
      fileReader.readAsArrayBuffer(file);
    }
  );
  return frObs;
};