gecko-dev/dom/serviceworkers/test/server_file_upload.sjs

const CC = Components.Constructor;
const BinaryInputStream = CC(
  "@mozilla.org/binaryinputstream;1",
  "nsIBinaryInputStream",
  "setInputStream"
);
const BinaryOutputStream = CC(
  "@mozilla.org/binaryoutputstream;1",
  "nsIBinaryOutputStream",
  "setOutputStream"
);

function handleRequest(request, response) {
  var bodyStream = new BinaryInputStream(request.bodyInputStream);
  var bodyBytes = [];
  while ((bodyAvail = bodyStream.available()) > 0) {
    Array.prototype.push.apply(bodyBytes, bodyStream.readByteArray(bodyAvail));
  }

  var bos = new BinaryOutputStream(response.bodyOutputStream);
  bos.writeByteArray(bodyBytes, bodyBytes.length);
}