That's an old Mac archival format. They can do all kinds of weird things when they show up nowadays. They'll try to open in Text Edit or Quicktime may want to open it thinking it's a video. Which some can be, but machines are stupid left to their own.
-Open with Stuffit Expander or other Mac compression/decompression app via File> Open or dragging into
-Try changing file extension to .ISO
-If you have Toast you can try mounting. I think it's Shift + Comm + M.
-See if it shows up in Disk Utility and try mounting it from there
-Tell clients you aren't a farmer and their shit is busted.