RAR archive of 284 KB named M5A78L-M_LX3_PLUS, corresponding to the ASUS M5A78L-M LX3 PLUS micro-ATX motherboard naming. What the archive contains is not stated in the name; verify after extraction whether it is a boardview, schematic or BIOS file. Boardviews open in tools like OpenBoardView for part lookup and net tracing, while firmware is written to the SPI chip with a programmer. Note that the LX3 PLUS is a distinct variant from the LX PLUS, so match the exact model and revision on the PCB.
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.