|Just a hunch. Add .jpg or .jpeg to the end of the files names. Maybe they come from a source that did not added the file format and Preview on OSX can recognize them. It happens with all kind of files on Macs that comes from not recognized origins.|
The Finder will ask you something like "Would you like to Add the JPG file extension to this file", choose Add.
Also you could select the file and choose "get info" and expand "Name & Extension". It should have a file format if not add one.