Wordpress : Guetzli upload prefilter plugin - php

Question

I'm trying to make my first WordPress plugins, and i have a issue.
I want to make a image compressor on upload in WordPress. I use the prefilter option to work on image before the upload is done with WordPress native upload function.
The fact is that the image upload, but without being compressed. But all seems good, and when I take the request sent and copy it in my computer's terminal, it works.
The server I use for now is my localhost, powered by xampp. Another similar function with pngquant works properly, so I don't think it's a server issue.
Here's my code :
function compressed_image_upload($file){
$ext = pathinfo($file['name'],PATHINFO_EXTENSION);
/*Use of guetzli*/
$allowed = array("jpg", "jpeg");
if(in_array($ext,$allowed)) {
echo "<pre>";
var_dump($file);
echo "</pre>";
$guetzli_path = get_home_path()."guetzli"; /*Preparation of guetzli command*/
if(is_executable($guetzli_path)) {
echo "guetzli executable : yes \n";
$request = $guetzli_path." ".$file['tmp_name']." ".$file['tmp_name'];
exec($request, $output, $return); /*Execution of guetzli*/
echo "<pre> I'm there ! \n";
echo "request : $request \n";
echo "output : $output \n";
echo "return : $return \n";
echo "</pre>";
}
}
return $file;
}
register_activation_hook( __FILE__, 'compressed_image_upload' );
And there is the results. As I said earlier, my image is upload, but as if the exec line did nothing.
result of this part of code
Any idea ? (First post on slackoverflow, be merciful if this question is not clear enough ^^. I'd gladly add informations if its lacks).
nb : guetzli works like this : "executable_path input_file_path output_file_path". In my case, i want it to erase the temporary file so it can be used by the next step of upload.