Add *upload* folder as static file folder.

upload-as-static-folder.py

import os
from flask import Flask, send_from_directory

app = Flask(__name__)

@app.route('/upload/<path:filename>')
def send_uploads(filename):
    print filename
    return send_from_directory(os.path.join(app.root_path, 'upload'), filename)

if __name__ == '__main__':
    app.run(host='0.0.0.0', debug=True)