It was a base64 encoding. You could decode it via your browser by converting it to a data URI.
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABkAAAAQCAYAAADj5tSrAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAC8SURBVDhP7ZLBDcMgDEU9lwdiHqZhGYb5tR1oibGlNJWqHvokDoTE336E8AV+N6QVAlFBG/sVO+OKPvbKvUlakRBGXSsZDYUIxaXf1HUUY59i4fuEeYh9oFrGcgoiLfbMjyHEIb2CTy8fnZ8KbMpiVcplXXuXTlmiSklCOipL0alqLtfmqixTpQQhI+CK76eyXJWSh6xfzJ9gqzKUMct5rErJL34q0iVTtSqFglZtwrCBF5cv/hP+IW8APAC6paINEHpT3QAAAABJRU5ErkJggg==
iVBORw = PNG so it was an image.