RAR archive of about 4.2 MB for the Apple iPhone 4. No further details such as board number, carrier variant or content type appear in the name. Packs of this sort normally hold logic board schematics or boardview files used to locate components, trace rails from the battery and PMIC, and diagnose shorts or open lines. Note that the iPhone 4 existed in GSM and CDMA versions with different boards, so compare the board markings with the documentation before relying on it.
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.