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Tabmorph

How to check whether a file converter uploads your files

Two tests anyone can do in a minute, what an upload looks like, and the warning signs to look for before you drop a private file into a website.

The short answer: load the converter, turn off Wi-Fi, and try converting a file. If it still works, nothing is being uploaded. For a closer look, open your browser’s Network tab and check for a request about as large as your file.

Test 1: the Wi-Fi test

  1. Open the converter page and wait a few seconds for it to finish loading.
  2. Turn off Wi-Fi or unplug your network cable.
  3. Convert a file that isn’t private.

If you get the converted file, the work happened in your browser. If you get an error, a spinner that never ends, or a “check your connection” message, the site needs its server to convert, so your file would be uploaded.

Test 2: the Network tab

  1. Open the converter, then your browser’s developer tools: F12, or ⌥⌘I on a Mac. Choose the Network tab.
  2. Clear the list, then add a test file and convert it.
  3. Type method:POST in the filter box (Chrome, Edge, Brave and Firefox). Uploads are almost always POST or PUT requests, so also try method:PUT.
  4. Click each request and look at its request headers and Payload. An upload has a Content-Length close to your file’s size, or a payload that names your file.

Small POST requests of a few hundred bytes are usually analytics or ads, not your file. Large GET requests are the page downloading code or images to you. Also check the WS filter: a site can stream a file over a WebSocket instead.

Warning signs, without any tools

  • An “Uploading…” progress bar before the conversion starts.
  • A queue or wait (“your file is being processed”), or a result sent by email.
  • A per-file size limit such as 100 MB, or a daily limit on conversions.
  • A note that files are deleted from the servers after a few hours.
  • A download link that points to the site’s own servers rather than a file made in your browser.

None of these prove misuse; plenty of upload-based converters handle files responsibly. But they do mean your file leaves your device, which matters for scans, IDs, contracts and private photos.

What Tabmorph does

Tabmorph converts files in your browser, and you can check it with both tests above. The converter also shows how many network requests it made since you added files. The only request the page sends is a tiny count of which converter ran, with no file data, as explained in how it works and the privacy page.

Last updated October 17, 2026.

Questions

A converter says “files are deleted after one hour”. Is that uploading?

Yes. Files can only be deleted from a server if they were sent to one. The promise is about what happens afterwards, not whether your file leaves your device.

The page made network requests while converting. Does that mean it uploaded my file?

Not necessarily. In-browser converters often download their own code, such as a decoder, the first time you use a format. Those are downloads to you. An upload is a request sending data out that is about as large as your file.

Is the Wi-Fi test enough on its own?

It is the most reliable quick test: a converter that still works with no connection can’t be sending your file anywhere at that moment. Load the page first and wait a few seconds, so its code is ready before you go offline.