Sending POST request from JS to PHP - $_FILES is NULL, any ideas why?

I am sending a POST request from a native JS file to a PHP file.

At some point, I was able to access my $_FILES var normally, without any problems ; however, I added a second param (extension) and now it simply is NULL.
I was able to take the file from $_FILES and save it in my server.

var_dump($_POST) prints the [Object FormData], but var_dump($_FILES) prints NULL (not empty).

Any ideas? Thank you very much

the JS file :

 const formData = new FormData();
 formData.append("file", droppedFile);

     
//post request
const saveFile = await fetch("../controller/save-file.php", {
    method: "POST",
    headers: {
          "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8",
    },
    body: `formData=${formData}&extension=${droppedFile.name.split(".").pop()}`
}).then((response) => response.json());

the PHP file :

<?php 

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: GET, POST');
header('Access-Control-Allow-Headers: X-Requested-With');

ini_set('file_uploads', 'On');
ini_set('upload_max_filesize', '1GB');
ini_set('post_max_size', '1GB');

$path = "../models/";

if(isset($_POST)){
   var_dump($_FILES); // PROBLEM HERE

    // $location = $path . 'fbx/' .$filename;  
    // if(!file_exists($location)){
    //    if(!move_uploaded_file($_FILES['file']['tmp_name'], $location))
    //    {
    //      echo json_encode("File was not saved");
    //      exit(1);
    //    }
    //    echo json_encode("file was created");
    // }
    // else{
    //    echo json_encode("file already exists");
    // }
}
else{
    echo json_encode("no POST data");
}

Try this:

 const formData = new FormData();
 formData.append("file", droppedFile);
 formData.append("extension",${droppedFile.name.split(".").pop());
     
//post request
const saveFile = await fetch("../controller/save-file.php", {
    method: "POST",
    body: formData
}).then((response) => response.json());

 $_POST['extension'] = ....
 $_FILES = ....