Compressed RAR package, 849 KB, labelled 915P-B with version 1.0, which points to a board built on the Intel 915P chipset at revision 1.0. The name gives no manufacturer or content type, so expect a schematic or similar board documentation inside. Open it with 7-Zip or WinRAR. Then compare the revision silkscreened on the PCB with version 1.0 before trusting component values, rail names or connector pinouts, since later revisions often move parts around.
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 @cyberlanelectonik. 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)