void handleFileUpload(){ // upload a new file to the SPIFFS

  HTTPUpload& upload = server.upload();
  if(upload.status == UPLOAD_FILE_START){
    String filename = upload.filename;
    if(!filename.startsWith("/")) filename = "/"+filename;
    Serial.print("handleFileUpload Name: "); Serial.println(filename);
    fsUploadFile = SPIFFS.open(filename, "w");            // Open the file for writing in SPIFFS (create if it doesn't exist)
    filename = String();
  } else if(upload.status == UPLOAD_FILE_WRITE){
    if(fsUploadFile)
      fsUploadFile.write(upload.buf, upload.currentSize); // Write the received bytes to the file
  } else if(upload.status == UPLOAD_FILE_END){
    if(fsUploadFile) {                                    // If the file was successfully created
      fsUploadFile.close();                               // Close the file again
      Serial.print("handleFileUpload Size: "); Serial.println(upload.totalSize);
      server.sendHeader("Location","/html/success.html");      // Redirect the client to the success page
      server.send(303);
      Serial.print("User-specified file path: ");
      Serial.println(server.arg("file_path"));
    } else {
      server.send(500, "text/plain", "500: couldn't create file");
    }
  }
}