Comment by throwitaway1123

Which browsers have you tested this in? I ran the feature detection script from the Chrome docs and neither Safari nor Firefox seem to support fetch upload streaming: https://developer.chrome.com/docs/capabilities/web-apis/fetc...

  const supportsRequestStreams = (() => {
    let duplexAccessed = false;
  
    const hasContentType = new Request('http://localhost', {
      body: new ReadableStream(),
      method: 'POST',
      get duplex() {
        duplexAccessed = true;
        return 'half';
      },
    }).headers.has('Content-Type');
  
    return duplexAccessed && !hasContentType;
  })();

Safari doesn't appear to support the duplex option (the duplex getter is never triggered), and Firefox can't even handle a stream being used as the body of a Request object, and ends up converting the body to a string, and then setting the content type header to 'text/plain'.