RAR archive of 1.19 MB named m540gmb, with no brand or board number listed. The name hints at an M540 motherboard document, though the maker and revision cannot be confirmed from the file name and should be read from the contents after extraction. Archives of this kind usually hold a schematic PDF, sometimes with a boardview, which a technician uses to trace power sequencing, identify components and check suspected short circuits. Compare the board number printed on the PCB with the document before using it.
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 @generalschematics. 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.