Xiaomi M3 archive in 7z format, about 14.4 MB, with no further detail in the name about its contents. M3 most likely refers to the Mi 3 handset, but check the files themselves to confirm the exact device and board revision. The larger size suggests several documents, images or possibly firmware pieces bundled together, so extract it with 7-Zip and sort the material before use. Service files of this kind support tracing power rails, locating components and test points, and planning reflashing or board-level repair.
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 @esquematicosdecelulares. Enginerys does not host this file.
How to open a .7z file
High-ratio compressed archive, often used for large BIOS collections.