use jschardet in browser

test-jschardet.js

// load jschardet js lib
// $.getScript('https://cdnjs.cloudflare.com/ajax/libs/jschardet/1.4.1/jschardet.min.js')

var xhr = new XMLHttpRequest(), blob;

xhr.open("GET", "http://127.0.0.1:5000/api/uploads/404/blob", true);
// Set the responseType to blob
xhr.responseType = "blob";

xhr.addEventListener("load", function() {
    if (xhr.status === 200) {
        console.log("Image retrieved");

        var blob = xhr.response;
        var reader = new FileReader();
        reader.addEventListener("loadend", function() {
            console.log(jschardet.detect(reader.result))
        });
        reader.readAsBinaryString(blob);
    }
}, false);
xhr.send();

https://github.com/aadsm/jschardet