Compact 1000 KB RAR archive for the MSI MS-7403 board. The file name carries nothing beyond the MSI board number, so the contents are unknown and could be a schematic or a firmware image. MS-7xxx is MSI's numbering for desktop motherboards, and the code is usually printed on the PCB near the PCI slots. If a BIOS dump is included, check that its size matches the capacity of the flash chip and save the original contents with a programmer before reflashing.
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 @IT_SERVICE_WORLD. 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)