How do i create a progress bar with flask?

I'm developing a web interface by using flask. The user log and reach an html page where a file can be upload. This file will be save and parse by an internal program (no relation with my problem)

I use a File Field in a Flask Form:

class MyForm(FlaskForm):
    file = FileField("DROP YOUR FILE HERE", validators=[DataRequired()])
    submit = SubmitField('Submit')

I call it in my main programm:

@app.route('/upload', methods=['GET', 'POST'])
@login_required
def uploadfile():
    form = MyForm()
    if form.validate_on_submit():
        file = form.file
        file.data.save(os.path.join("path/to/", secure_filename("filename")))
        flash("File added")
        return redirect(url_for('uploadfile'))
    return render_template("upload.html",form=form)

and here is the body of my html page:

<form action="{{url_for('uploadfile')}}" method="post" enctype="multipart/form-data" novalidate>
    {% with messages = get_flashed_messages() %}
        {% if messages %}
            {% for message in messages %}
                <p class="error">{{ message }}</p>
            {% endfor %}
        {% endif %}
    {% endwith %}
    {{ form.csrf_token }}
    {{form.fichier}}
    {% for error in form.fichier.errors %}
        <span style="color: red;">[{{ error }}]</span>
    {% endfor %}
    <p>{{ form.submit() }}</p>
</form>

The problem is uploading and saving a lot of data take lots of time. So i want to add a progress bar but i don't know how to get the download progress and send it to my html progress bar.

This is why i ask your help. If you have any solution i will take it.

Thanks you