[Answer]-Django: Does filepicker.io use passthrough or direct approach?

From their home page.

Your files are stored in your existing S3 bucket. We handle all the complicated javascript and backend APIs to give you a clean and elegant way of saving files directly to your S3 bucket.

Since they fallback to Iframe embedding, It’s likely that the “upload” will go through their server before ending up in your bucket. But that’s bandwidth that you don’t have to pay for.