Small 152 KB RAR archive named only after the T420, presumably the ThinkPad model of that name, though the contents are not described. A package this compact usually holds a BIOS or EC firmware dump rather than a full schematic. Open it with WinRAR or 7-Zip and check the image size against the flash chip on the board, since a dump that does not match the chip capacity will not fit. Always read and save the original chip contents with your programmer before reflashing, so the board can be restored if needed.
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 @hrtechno. 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.