set a "default" directory in a function when asking for a file

[Niels Felsted Thorsen <nift@maclisp.org>]

Hi

I have a function like this:

(defun testfun (file)
  (interactive "fFile: ")
  (message file))

But it will ask for the file based on, as I understand it, the value
of "default-directory" which is buffer-local.

Is there a way to specify default-directory just for this function? So
it will always start with fx:

File: ~/foo/bar/

when asking for the file.