In this case, since it was a dump from a cartridge you actually own, it already comes with the header included and that's why all you need to do is change the extension to .3dz.
But keep in mind that roms downloaded online often come in .3ds format (with no header) and you need to inject your own private header into it aside from also changing the extension to .3dz.
You may also find roms online already in .3dz format with an header included but do not use those or change the header to your own private header since using public headers (used by more than 1 person) will get you banned.