3DS Max hasn't used the 3DS format in over 2 decades. Like OBJ, it's an out of date legacy format.
Assuming 3D Studio Max
Post reminds me of me in my early days when I had no net access and file loading was still beyond me. I used GIF for everything because I had working GIF code someone had given me on FIDONet.
So it sounds like someone is clinging to old code, or old tools. With 8.3, maybe it's an old DOS tool! Could be that the code assumes 8.3 for the files, and has a hard coded char array check for the extension.