3DS is the original (legacy) file format for Kinetix' 3D Studio and 3D Studio MAX product line (now part of AutoDesk). It contains the 3D model data for the complete scene.
See also the successor format, MAX.
3DS files begin with bytes
0x4D 0x4D. (Note that many TIFF files begin the same way, though 3DS is not related to TIFF.)