React Native HTTP request with FormData including an image

My goal is to send a photo from the local device to server.

I used import { launchImageLibrary } from "react-native-image-picker"; to select an asset called photo and used

const resp = await fetch(photo.uri);
const blob = resp.blob();
formData.append('regPhoto', blob);
registerUser(formData);

to append this as a blob to the form data. I checked that the await fetch call is returnin 200 with meaningful data. Then this formData sent with XMLHttpRequest API:

function registerUser(formData: FormData) {
  cl("registerUser");
  return new Promise((resolve) => {
    const xhr = new XMLHttpRequest();
    xhr.open("POST", url + "registerUser");
    xhr.onreadystatechange = () => {
      const resObj = onReadyStateChange(xhr);
      if (resObj) {
        resolve(resObj);
      }
    };

    xhr.send(formData);
  });
}

Two things happen, the server receives the request without the image and processes it without the image. Which is success to some degree. At the same time, there is a JSON response at the client (android emulator) that reads: Unrecognized formdata part. I am not sure where this is coming from actually.

Does anybody have an idea whats going on here? Just wanted to upload an image to my server via formData.

I tried other methods of appending the photo to the formData but I get the same error.