If you downloaded the latest toastman or kong then just go to upgrade show it where the .bin is and "reset to defaults" then let it load. This works for both dd-wrt and toastman tomato.
Then do the peacock 30/30/30 reset and let it run for 2 mins for nvram to settle.

Older firmware version needs to specifically have the .bin or .trx but there it is better to go from old tomato to small dd-wrt and then to big dd-wrt and same for reverse, small then to big.

Changing the extension works but could cause issues or something.

Make sure you flash the new firmware of any type you like then close firefox / safari / web browser and open it again form a fresh start. The browsers like to keep info that could irritate you so best is the start firefox in private mode where it does not keep any settings / clears all settings you just used when the browser closes.

This avoids a lot of issues.
Also if you use tftp flash in recovery mode make sure you flash then 30/30/30 and then once working flash the same firmware again from the gui and 30/30/30, this is overkill but has helped a few times with odd behavior.