如何确认上传“直接”到S3，而不先通过后端服务器进行代理？ （Python Flask示例）

I am curious how to tell the difference between using Fineuploader to upload files to S3 using the traditional method of proxying the files first through the backend versus doing a direct upload from the client. Here is the backend code I have in place that I retrieved from the Fineuploader Github and tweaked slightly to my usecase.I am curious how to tell the difference between