This is a basic example showing how to use Flask, Flask-WTF and Boto to automatically upload user files to S3, bypassing storage on the server.
Update config.py with your Amazon details.
The most efficient way of handling the downloads is likely to be just upload the file to the local webserver to enable a quick as possible response to the user, then adding the 'upload to S3' task to a message/task queue that runs in the background as the server can manage.