File uploading with php: problem with $_POST['filename']

Question

I want to upload files from my Android app to my php server (through a https upload URL)

Here is my php code :

$path = "../uploads";
        
        if (!file_exists($path)) {
            mkdir($path, 0777, true);
        }

 $filePath = $path."/".$_POST['filename'];

move_uploaded_file($_FILES['uploaded_file']['tmp_name'], $filePath)

The content of $_FILES['uploaded_file'] is:

{"name":"scan.gltf","type":"","tmp_name":"\/srv\/data\/tmp\/phpmEM42e","error":0,"size":5071}

So everything seems ok as I get the name of my file and its correct size.

But when I log $_POST['filename'], I get a null string.

How is that possible ? How to solve the problem ?

---

Here below my Java code from my Android app but I don't think that the problem comes from there (perhaps I am wrong..):

FileInputStream fileInputStream = new FileInputStream(sourceFile);
URL url = new URL(urlString);
// Open a HTTP  connection to  the URL
// TODO HttpURLConnection or HttpsURLConnection ???
conn = (HttpURLConnection) url.openConnection();  
conn.setDoInput(true); // Allow Inputs
conn.setDoOutput(true); // Allow Outputs
conn.setUseCaches(false); // Don't use a Cached Copy
conn.setRequestMethod("POST");
conn.setRequestProperty("Connection", "Keep-Alive");
conn.setRequestProperty("ENCTYPE", "multipart/form-data");
// TODO Space or no space before 'boundary=' ?
conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
conn.setRequestProperty("uploaded_file", fileUrl); 
dos = new DataOutputStream(conn.getOutputStream());
dos.writeBytes(twoHyphens + boundary + lineEnd);
// TODO Space or no space before 'filename=' ?
dos.writeBytes("Content-Disposition: form-data; name=\"uploaded_file\"; filename=\"" + fileUrl + "\"" + lineEnd); 
dos.writeBytes(lineEnd);
// create a buffer of  maximum size
bytesAvailable = fileInputStream.available();
bufferSize = Math.min(bytesAvailable, maxBufferSize);
buffer = new byte[bufferSize];
// read file and write it into form...
bytesRead = fileInputStream.read(buffer, 0, bufferSize);
                
while (bytesRead > 0) {
dos.write(buffer, 0, bufferSize);
bytesAvailable = fileInputStream.available();
bufferSize = Math.min(bytesAvailable, maxBufferSize);
bytesRead = fileInputStream.read(buffer, 0, bufferSize);
}
                
// send multipart form data necessary after file data...
dos.writeBytes(lineEnd);
dos.writeBytes(twoHyphens + boundary + twoHyphens + lineEnd);
                
// Responses from the server (code and message)
int serverResponseCode = conn.getResponseCode();
String serverResponseMessage = conn.getResponseMessage();