First of all sorry for my bad English it is not my firs language.
Ok let`s get on with the problem. I was trying to downgrade from OpenWRT to the stock firmware of my linksys ea8500 because i wanted to install DD-WRT. I did the following things:
1.I went to the OpenWRT website and in search i wrote my routers module and downloaded the
"Firmware OEM Stock URL"
2.I opened my web browser and went to the "OpenWRT" GUI then went to system>Backup/Flash Firmware then browsed for the .img and pressed the "flash image" button and then it said
"The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform."
You can bypass the image format verification in OpenWrt, by "forcing" the flashing.
In master it works directly in LuCI. In 18.06 you need to use sysupgrade from the command line and use option "-F" to force the flashing despite failing the image format verification.
Note that you need to be sure about the image that you are flashing, as you bypass the normal check...
You need to download the needed image to the router e.g. by using wget, or by scp or whatever you like.
Save the image to /tmp
sysupgrade -F -n /tmp/imagefilename