use strict;
use warnings;
use File::Basename;
use File::Spec;

my $attach = "file.png";
my $filename = File::Basename::fileparse($attach);
my $tempfile = File::Spec->catfile("tempdir", $filename);
my $new_filepath = File::Spec->catfile("tempdir", "allfiles.png");

rename $tempfile, $new_filepath
    or die "Cannot rename file $tempfile to $new_filepath: $!";