@app.route('/get_file', methods=["GET"])
def get_file():
    filepath = request.args.get('filepath')
    dirname, fname = os.path.split(filepath)
    return send_from_directory(dirname, fname, as_attachment=True)

{% for path in data %}
<img class='lazy' data-src="{{ url_for('get_file', filepath=path) }}">
{% endfor %}

import asyncio

@app.route('/get_file', methods=["GET"])
async def get_file():
    filepath = request.args.get('filepath')
    dirname, fname = os.path.split(filepath)
    await send_from_directory(dirname, fname, as_attachment=True)