@app.route('/upload', methods=['GET', 'POST'])
def upload():
    if request.method == 'POST':
        for f in request.files.getlist('file'):
            f.save(os.path.join(app.config['UPLOADED_PATH'], f.filename))
return render_template('your template to render')
Dropzone.js не позволяет Flask отображать шаблон