It seems as if windows clients (IE) submit filenames with backslash as directory separator.
(no surprise :-).

But the attachment plugin translates these backslashes to underscore, making the
whole path a filename.

This little hack fixed the backslash problem, although I wonder if that
really is the problem?
(Everything works perfectly from linux clients of course. :-)

	sub basename ($) {
		my $file=shift;

		$file=~s!.*/+!!;
		$file=~s!.*\\+!!;
		return $file;
	}

Should probably be `$file=~s!.*[/\\]+!!` :-)