MSI MS-6599 archive in zip format, 2.10 MB. No revision, chipset or file type appears in the name, so the contents have to be checked after extraction. A package of this size typically carries a BIOS image or a scanned service document for the board. The MS- code is MSI's in-house board numbering and is printed on the motherboard itself. Read that code and the revision off the PCB before using any schematic for rail tracing or any image for reprogramming the firmware chip.
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 .zip file
Compressed archive that every operating system opens without extra software.
Built-in OS tools7-Zip