I tried to upgrade an el cheapo TP-LINK ADSL router just now and ran into the following error:
ERROR: FAIL TO UPDATE DUE TO... The uploaded file was not accepted by the router.
TP-LINK does warn you to check that you are uploading the correct firmware: right model and right "version" number. In this case, it was correct. However, the upgrade failed nonetheless. The solution was to not upgrade to the latest version directly, but to proceed sequentially in chronological order. I first updated the 2014 release, then the 2015, and finally the 2016. These proceeded fine and the router-modem rebooted perfectly.
Hope this helps somebody!