BIOS / EFI firmware for an Apple logic board numbered 820-01814, packed in a 633 KB RAR. Apple marks its logic boards with 820-xxxxx numbers, and the matching model is not stated in the name. The small archive likely holds a compressed flash image for writing to the board's SPI chip with a programmer. Read the original chip first, because it holds the serial number and other unique data. Confirm that the board number on the PCB matches exactly and that the image size equals the chip capacity.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @biosarchive. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.
Back up your own chip before writing any dump, and match the full board number and revision. Flashing guide →