@DeeGee 

What is the firmware file that you tried to upload?

If you look at the sticker label of your router, what is the country code that is on there? Would be something like (US) or UK) etc.

@DeeGee 

We're trying to use the US version, which matches our hardware.  I don't have the full file name in front of me, but it is the current .bin, downloaded from TP Link support.

@DeeGee 

Try downloading the firmware one more time here: https://static.tp-link.com/Archer_C50(US)_V2_160801_1472173209786e.zip

That is for the Archer C50 v2 (US) region.

Before doing the update, the factory default the router by holding the reset for 15 seconds. Then connect a computer to the router with an Ethernet cable, and log in.

Try to upload the .bin file again, if you still get the same error, use a different web browser. If that still fails, use another computer if possible.