Preview spooled PostScript.
Interactively, when you use a prefix argument (C-u), the command prompts the
user for a file name, and saves the spooled PostScript image in that file
instead of saving it in a temporary file.
Noninteractively, the argument FILENAME is treated as follows: if it is nil,
save the image in a temporary file. If FILENAME is a string, save the
PostScript image in a file with that name. (fn &optional FILENAME)