Ok you have to see if the downloaded file is not an archive (.ace, .rar, zip archives are recognized automaticly by pj64 and goodn64), also you have to see it's length, if it is a rom it has to be 8,16 or 32Megs. Good luck.
i think the *.rom extension dumps are usually non working roms.I have about 200 roms and only the 10 of them have the extension *.rom.I don't know why the most usuall is *.v64 but the bad thing is i have to say that 80% you d/l a bad rom
one last thing.I don't want to be like i am postin rom sites i just want to help.So here is my question (and answer for me if is a bad rom)....Did you downloaded it from vi**r's roms? .Cause if you did i know for sure(about donkey kong) that is a bad rom
Most are .v64 because they were dumped using a product called v64 or doctor64 (Can't remember which one though) but that's the reason for the .v64 extensions.