The DSi’s native update routine does not read firmware files from the SD card slot. The only official update path is via Nintendo’s servers.
If you are looking for firmware files (like bios7.bin , bios9.bin , or firmware.bin ) for emulators (e.g., melonDS, No$GBA, or Delta), these are copyrighted files usually dumped from physical hardware. System Update | Support | Nintendo ZA Nintendo Dsi Firmware Download
Custom firmware allows you to run homebrew applications and games directly from an SD card. Required Tools The DSi’s native update routine does not read