EPSI is EPS with a device independent bitmap preview. EPSI is an all ASCII (no binary data or headers) version of EPS. EPSI provides for a hexadecimal encoded preview representation of the image that will be displayed or printed. EPSI files were documented by Adobe as a means of providing a preview for EPS files which would be cross-platform. In reality though DOS machines and Windows favour embedding TIFF or even Windows Metafiles in the PostScript. I don't know of any Mac packages that support EPSI files.
The Macintosh has always favoured EPSF files with the data fork of the file containing the PostScript code, and the resource fork containing a PICT preview of the file. This is how typically when you import an EPSF file into a program (Quark, Persuasion etc.) you get to see the PICT preview from the file. It is still possible to have an EPSF file without a preview though, in this case the imported artwork is displayed usually as a box with diagonal lines running through it. Note the term EPSF really means that the image is independant of an output device, in other words the image would need to be embedded inside other PostScript code to be usable (printable). The old command-lk files from the PS driver on the Mac are not EPSF files as they are produced for a specific output device (LaserWriter). Therefore they cannot be imported into other applications for printing. ESPF must be given resource number 256.