My problem is almost exactly like in this unsolved quastion.

Downloading starts properly, if route function was called directly:

HTML

<button type="button" onclick="window.location.href='{{ url_for( 'download') }}';">

Python

@app.route('/download', methods=['GET', 'POST'])
def download():
    # ...
    return send_file(data,
                     mimetype=mimetype,
                     as_attachment=True,
                     download_name='default' + '.' + ext)

But I’d like to do some additional work on JS side.
So onclick event triggers the function which, in its turn, tries to call flask route through ajax-request, but nothing happens. Downloading doesn’t start.

HTML

<button type="button" onclick="before_download()">

JS

async function before_download(ev) {
    let name = document.getElementById('name').value;
    // ...
    let response = await fetch('/download', {
        method: 'POST',
        headers: {'Content-Type': 'application/json; charset=utf-8'},
        body: JSON.stringify({name})
    });
    // return await response.blob(); // Do I need this line at all?
}

Python

@app.route('/download', methods=['GET', 'POST'])
def download():
    # ...
    return send_file(data,
                     mimetype=mimetype,
                     as_attachment=True,
                     download_name=request.get_json()['name'] + '.' + ext)

What should be returned from route function? Shall I await for it in JS?
Or maybe is there a special header for this kind of requests?

Thanks.