Android: java.io.FileNotFoundException when trying to upload file to PHP server using retrofit

Question

I want to upload multiple pdf files to my server. I am following this link for this purpose. My code is as following:

server_interface.java:

public interface server_interface {
    @Multipart
    @POST("server.php")
    Call<ResponseBody> uploadMultipleFilesDynamic(
        @Part List<MultipartBody.Part> files
    );
}

MainActivity.java:

server_interface api;

@Override
protected void onCreate(Bundle savedInstanceState) {
    List<MultipartBody.Part> parts = new ArrayList<>();
    for(Uri uri: list){
        parts.add(prepareFilePart(uri));
    }
    OkHttpClient.Builder httpClient = new OkHttpClient.Builder()
                .callTimeout(2, TimeUnit.MINUTES)
                .connectTimeout(2, TimeUnit.MINUTES)
                .readTimeout(3, TimeUnit.MINUTES)
                .writeTimeout(3, TimeUnit.MINUTES);

    Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(server_interface.JSONURL)
                .addConverterFactory(ScalarsConverterFactory.create())
                .client(httpClient.build())
                .build();

    api = retrofit.create(server_interface.class);
    Call<ResponseBody> uploadcall = api.uploadMultipleFilesDynamic(parts);
    if(uploadcall != null){
        uploadcall.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(@NonNull Call<ResponseBody> call, @NonNull Response<ResponseBody> response) {
            if(response.body() != null) {
                if (response.isSuccessful()) {
                    Toast.makeText(MainActivity.this, response.body().toString(), Toast.LENGTH_SHORT).show();
                }
            }
            @Override
            public void onFailure(@NonNull Call<ResponseBody> call, @NonNull Throwable t) {
                Log.d("response",t.toString());
            }
        });
    }
}
@NonNull
private MultipartBody.Part prepareFilePart(Uri fileUri) {
    File file = new File(fileUri.toString());
    Log.d("exists", String.valueOf(file.exists())); //shows false
    RequestBody requestFile =
                RequestBody.create(
                        MediaType.parse(getContentResolver().getType(fileUri)),
                        file
                );
    return MultipartBody.Part.createFormData("pdf", file.getName(), requestFile);
}

server.php:

<?php
    error_reporting(E_ALL);
    ini_set('display_errors', 1);
    header('Content-Type: text/plain; charset=utf-8');
    header('Access-Control-Allow-Origin: *');
    header('Access-Control-Allow-Methods: POST, GET, OPTIONS');

    if(count($_FILES) > 0){
        $location = array();
        $folder="attachments/";
        for($i=0; $i<count($_FILES['file']['name']); $i++){
            if(!file_exists($folder.$_FILES['file']['name'][$i])){
                $moved = move_uploaded_file($_FILES['file']['tmp_name'][$i],$folder.$_FILES['file']['name'][$i]);
                if($moved){
                    if($i != count($_FILES['file']['name'])-1)
                        array_push($location,$folder.$_FILES['file']['name'][$i].';');
                    else
                        array_push($location,$folder.$_FILES['file']['name'][$i]);
                }
                else{
                    array_push($location,"Unable to upload");
                }
           }
           else{
                if($i != count($_FILES['file']['name'])-1)
                    array_push($location,$folder.$_FILES['file']['name'][$i].';');
                else
                    array_push($location,$folder.$_FILES['file']['name'][$i]);
           }
        }
        if(count($location) != 0){
           if(!in_array("Unable to upload",$location))
               echo json_encode($location);
           else
               echo "An attachment wasn't uploaded. Please try again. If issue persists, contact ITC/NFR/MLG";
        }
        else
           echo "Unable to upload";
    }
?>

When I execute my code, I keep getting an error in my onFailure():

java.io.FileNotFoundException: /document/document:1000011388: open failed: ENOENT (No such file or directory)

How do I fix this issue?

EDIT: This is how I am getting the uri

ActivityResultLauncher<Intent> intentresultlauncher = registerForActivityResult(
    new ActivityResultContracts.StartActivityForResult(),
        result -> {
            if (result.getResultCode() == Activity.RESULT_OK) {
                Intent data = result.getData();
                if(data != null){
                    Uri pdf_data = data.getData();
                    prepareFilePart(pdf_data);
                }
            }
        }
);
button.setOnClickListener(view -> {
    Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
    intent.addCategory(Intent.CATEGORY_OPENABLE);
    intent.setType("application/pdf");
    intentresultlauncher.launch(intent);
});