DuetWebControl.bin was used with firmware 1.18 and earlier, when the web server ran on the WiFi module. With 1.19 and later, the web server runs on the Duet main processor instead and the web files are fetched from the SD card. It's because of this big change that the upgrade from 1.18 or earlier to 1.19 or later on the Duet WiFi is more complicated than firmware upgrades normally are.