Find FILENAME, guessing a default from text around point.
If `ffap-url-regexp' is not nil, the FILENAME may also be an URL.
With a prefix, this command behaves exactly like `ffap-file-finder'.
If `ffap-require-prefix' is set, the prefix meaning is reversed.
See also the variables `ffap-dired-wildcards', `ffap-newfile-prompt',
and the functions `ffap-file-at-point' and `ffap-url-at-point'.
(fn &optional FILENAME)