How to check that a site does not upload your files

1. Open the Network panel

On a computer, open the site and press F12 (on a Mac ⌥ ⌘ I). Choose the Network tab. Every request the page makes from now on appears in this list.

2. Open a file

Use a file you do not mind about for the first try. Open it on the site the way you normally would.

3. Read the list

• Requests for scripts, styles, fonts and images of the site itself are normal.
• An upload looks like a request with the method POST or PUT and a size close to your file’s. Click it and open Payload to see what was sent.
• Watch the Fetch/XHR and WS filters: that is where uploads and live connections show up.

On OsonKit, opening a file adds at most the decoder for its format — like pdf.worker.js — and nothing with your file in it.

An even simpler test

Load the site, then switch off Wi-Fi and open a file. A site that needs a server to read your file stops working. OsonKit keeps working, because the file never needed to go anywhere.