Never update firmware over Wi-Fi. A signal drop during the flash will ruin the router.
Firmware usually downloads as a .zip . Extract it to find the .bin file before uploading. dir853 et a3 firmware update link
You can download the latest firmware for DIR-853 ET A3 from the following links: Never update firmware over Wi-Fi
Custom builds for the DIR-853 A3 (Platform: ramips/mt7621) can be generated or downloaded via the OpenWrt Firmware Selector Installation Images: dir853 et a3 firmware update link