Need assistance with defining own Oservable for validation of file upload in Angular 17

I'm following a tutorial for file upload in Angular 17, but in the tut, they use Observable. create, which is deprecated in Angular 17. I have read the docs on implementing the new system, but it does not work. Also, in the tut, they use a type Observer<{ [key: string]: any }>, but I cannot assign a type to null, which is this method called observer.next(null);

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 fileObserver = new Observable((observer: Observer<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.toString(16);
}
switch (header) {
case '89504E4':
isValid = true;
break;
case 'ffd8ffe0':
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 fileObserver;
};

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