RAR archive for the H81M-C motherboard, revision R2.0, about 209 KB in size and with no brand stated. The H81M-C name points to an Intel H81 chipset microATX board, a model name used by ASUS, and the small size suggests schematic or boardview material rather than firmware. Unpack it with 7-Zip or WinRAR to see what it holds. Check that your PCB is marked revision 2.0, since component placement and rail layout can change between revisions of the same model.
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 @boardview. 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.
7-ZipWinRARThe Unarchiver (macOS)