The .001 extension indicates this is the "header" or beginning of the sequence.
Large installers broken into smaller chunks for easier downloading. How to Open This File
Right-click on the .001 file specifically.
Because this file is part of a set, it cannot be opened or extracted on its own. You will need all subsequent parts (e.g., .001 , .002 , .003 ) in the same folder to successfully access the contents. Technical Analysis 7-Zip Split Archive (Part 1).
Large video files or stock photo collections (similar codes like "REB1MC" are used in photography archives).
Large textures or "re-baked" (REB) game files.
Ensure you have every numbered file in the sequence (Reb1MF.7z.001, Reb1MF.7z.002, etc.) saved in the same directory.