Hi again Geoff,
Without going into a bunch of mumbo jumbo, much of which I don't even understand...
File extensions are very important as the application accessing the file will look for certain file extension associations and depending on the file extension deal with the file accordingly.
.jpg, .jpeg, .png, .gif are all standard graphic file extensions that most browsers know exactly what to do with. .jpg72 causes some (if not all) browsers to stop and say Huh