use Image::Info qw(image_info);
my $filename = "file5.jpg"; # actually a PNG
my $info = image_info($filename);
if (my $error = $info->{error}) {
    die "Can't parse image info: $error\n";
}
my $mime_type = $info->{file_media_type};
my $suggested_ext = $info->{file_ext};
print "mime type: $mime_type\n";
print "suggested extension: $suggested_ext\n";