Get blob from <img /> using Canvas. In that way you can append it to a FormData and make a POST with it.

get_blob_from_img.js

//let img = document.querySelector("img");

async function imgSourceToBlob(img) {
  let canvas = document.createElement('canvas');
  canvas.width = img.width;
  canvas.height = img.height;

  let ctx = canvas.getContext('2d');
  ctx.drawImage(img, 0, 0);

  let imageBlob = await new Promise((resolve) => {
    canvas.toBlob((blob) => resolve(blob));
  });

  return imageBlob;
}

/**
* let body = new FormData();
* let blob = await imgSourceToBlob(img);
* body.append(
*   'file',
*   new File([blob], `${Date.now()}_${user._id}_${index}`, {
*   type: blob.type,
* });
* ...
*/