chill-bundles/Resources/public/module/async_upload/downloader.js

var mime = require('mime-types');

var algo = 'AES-CBC';

var initializeButtons = (root) => {
    var
        buttons = root.querySelectorAll('a[data-download-button]');
    
    for (let i = 0; i < buttons.length; i ++) {
        initialize(buttons[i]);
    }
};

var initialize = (button) => {
    button.addEventListener('click', onClick);
};

var onClick = e => download(e.target);

var download = (button) => {
    var
        keyData = JSON.parse(button.dataset.key),
        ivData = JSON.parse(button.dataset.iv),
        iv = new Uint8Array(ivData),
        urlGenerator = button.dataset.tempUrlGetGenerator,
        hasFilename = 'filename' in button.dataset,
        filename = button.dataset.filename,
        labelPreparing = button.dataset.labelPreparing,
        labelReady = button.dataset.labelReady,
        mimeType = button.dataset.mimeType,
        extension = mime.extension(mimeType),
        key, url
        ;
    
    button.textContent = labelPreparing;
    
    window.fetch(urlGenerator)
        .then((r) => {
            if (r.ok) {
                return r.json();
            } else {
                throw new Error("error while downloading url " + r.status + " " + r.statusText);
            }
        })
        .then(data => {
            url = data.url;

            return window.crypto.subtle.importKey('jwk', keyData, { name: algo, iv: iv}, false, ['decrypt']);
        })
        .then(nKey => {
            key = nKey;

            return window.fetch(url);
        })
        .then(r => {
            if (r.ok) {
                return r.arrayBuffer();
            } else {
                throw new Error(r.status + r.statusText);
            }
        })
        .then(buffer => {
            return window.crypto.subtle.decrypt({ name: algo, iv: iv }, key, buffer);
        })
        .then(decrypted => {
            var 
                blob = new Blob([decrypted], { type: mimeType }),
                url = window.URL.createObjectURL(blob)
                ;
            button.href = url;
            button.target = '_parent';
            button.type = mimeType;
            button.textContent = labelReady;
            if (hasFilename) {
                button.download = filename;
                if (extension !== false) {
                    button.download = button.download + '.' + extension;
                }
            }
            button.removeEventListener('click', onClick);
            button.click();
        })
        .catch(error => {
            console.log(error);
        })
        ;
};

window.addEventListener('load', function(e) {
    initializeButtons(e.target);
});