In my HTML document, there is a image

```
<img src="https://www.gravatar.com/avatar/1d9c87a4d5cbcae4c76e867cb6861fa0?s=48&d=identicon&r=PG&f=1"/>

```

When open this document in browser, E.g. Chrome, we can find the `Content-Type` of this image is `"image/png"` in Chrome Developer Tools -> Network tab according to the HTTP response headers.

How can I get the `Content-Type` using JavaScript after the image loading. 

More specifically, what I need is a function like below.

```
/**
 * @param  {HTMLImageElement} img
 * @return {String}     the content type, e.g. "image/png", "image/svg+xml"
 */
getImgContentType(img) {
    // TODO
}

```