Compressed archive of 419 KB named M5A99FX_PRO, a model name used on an ASUS AM3+ desktop board, although the name gives no brand or revision. The contents are unspecified, so inspect them before use. If the archive carries a BIOS image, read and save the original flash chip before writing with a programmer, and match image and chip sizes. If it contains schematic or boardview data, compare the board revision on the PCB first.
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)