.smi files aren't around much anymore. They were Self Mounting Image files of disks made by older versions of Disk Copy or Stuffit for Mac OS 8.6 (or 9?) and earlier, and didn't transfer well unstuffed over the internet. You can try dragging it onto stuffit expander, or maybe even Disk Copy. I recall the only way I could mount some I had made, was to open them on the old PPC machine I used to make them. I vaguely recall another workaround--duplicate the file and change the extension from .smi to .img.