diff --git a/composer.json b/composer.json
index 6e3d6de..af61bc3 100755
--- a/composer.json
+++ b/composer.json
@@ -1,4 +1,12 @@
 {
+  "name": "romhackplaza/wp-plugin",
+  "type": "project",
+  "authors": [
+    {
+      "name": "Benjamin",
+      "email": "benjamin@cartememoire.org"
+    }
+  ],
   "autoload": {
     "psr-4": {
       "RomhackPlaza\\": "src/"
diff --git a/src/Extenders/Submissions.php b/src/Extenders/Submissions.php
index 8b27eae..fb45544 100644
--- a/src/Extenders/Submissions.php
+++ b/src/Extenders/Submissions.php
@@ -111,11 +111,15 @@ class Submissions extends Abstract_Extender {
                 'submissions-uploader',
                 ROMHACKPLAZA_PLUGIN_URI . '/assets/js/submissions/uploader.js',
                 [],
-                '20251106-68',
+                '20251106-77',
                 args: [ 'defer' => true, 'in_footer' => true ]
             )->enqueue()->add_localize(
                 '_romhackplaza_script_uploader',
-                [ 'submit_url' => admin_url( 'admin-ajax.php' ) ],
+                [
+                    'submit_url' => admin_url( 'admin-ajax.php' ),
+                    'reserve_nonce' => wp_create_nonce( Format::format_nonce_name( 'reserve_post_id' ) ),
+                    'set_favorite_nonce' => wp_create_nonce( Format::format_nonce_name( 'set_favorite_server' ) ),
+                ],
             )->enable_i18n();
 
         }
diff --git a/ts/submissions/class-reserve-post-id.ts b/ts/submissions/class-reserve-post-id.ts
index bb2e4ed..4c5fd2d 100644
--- a/ts/submissions/class-reserve-post-id.ts
+++ b/ts/submissions/class-reserve-post-id.ts
@@ -14,5 +14,41 @@ export class Reserve_Post_ID {
             callback();
             return;
         }
+
+        if( I.custom_post_type === undefined || I.custom_post_type === "unknown" ){
+            console.error( "The custom post type is missing" );
+            return;
+        }
+
+        this.do_request( callback );
+
+    }
+
+    private do_request( callback: () => any ){
+
+        jQuery.ajax({
+            type: "POST",
+            url: _romhackplaza_script_uploader.submit_url,
+            data: {
+                action: "reserve_post_id",
+                custom_post_type: I.custom_post_type,
+                _wpnonce: _romhackplaza_script_uploader.reserve_nonce
+            },
+            success: function( response: any ) {
+                if( response.success ) {
+
+                    I.reserved_post_id = response.data.post_id;
+                    jQuery( "#reservedPostID" ).val( I.reserved_post_id );
+                    callback();
+
+                } else {
+                    romhackplaza_manage_modal( romhackplaza_modal_submissions, "block", __( "Submit error", 'romhackplaza' ), __( "Could not reserve a post ID. Please refresh the page.", "romhackplaza" ), "" );
+                }
+            },
+            error: function( response: any ) {
+                romhackplaza_manage_modal( romhackplaza_modal_submissions, "block", __( "Submit error", 'romhackplaza' ), __( "Error occurred while reserving a post ID. Please refresh the page.", "romhackplaza" ), "" );
+            }
+        });
+
     }
 }
\ No newline at end of file
diff --git a/ts/submissions/class-server-api.ts b/ts/submissions/class-server-api.ts
new file mode 100644
index 0000000..77d6484
--- /dev/null
+++ b/ts/submissions/class-server-api.ts
@@ -0,0 +1,50 @@
+import { __, I } from "./globals";
+
+export class Server_Api {
+
+    url: string|undefined;
+    id: number|undefined;
+
+    init( server_url: string, server_id: number ){
+        this.url = server_url;
+        this.id = server_id;
+    }
+
+    is_init(): boolean {
+        return this.url !== undefined && this.id !== undefined;
+    }
+
+    private build_url( call: string ): string {
+
+        let new_url: string = this.url + "upload.php";
+        if( call === "check_existence" )
+            new_url += "?checkExistence=true";
+
+        return new_url;
+    }
+
+    async check_file_existence( filename: string ): Promise<boolean> {
+
+        return new Promise( ( resolve, reject ) => {
+
+            const XHR: XMLHttpRequest = new XMLHttpRequest();
+            XHR.open( 'POST', this.build_url( 'check_existence' ), true );
+
+            let form_data: FormData = new FormData();
+            form_data.append( "filename", filename );
+            form_data.append( 'reservedPostID', I.reserved_post_id );
+            form_data.append( 'custom_post_type', I.custom_post_type );
+
+            XHR.onload = () => {
+                resolve( XHR.responseText === 'exists' );
+            }
+            XHR.onerror = () => {
+                reject( false );
+            }
+
+            XHR.send( form_data );
+
+        });
+
+    }
+}
\ No newline at end of file
diff --git a/ts/submissions/class-set-favorite-server.ts b/ts/submissions/class-set-favorite-server.ts
new file mode 100644
index 0000000..ec33606
--- /dev/null
+++ b/ts/submissions/class-set-favorite-server.ts
@@ -0,0 +1,34 @@
+import {__, I } from "./globals";
+declare const _romhackplaza_script_uploader: any;
+declare const romhackplaza_modal_submissions: any;
+
+export class Set_Favorite_Server {
+
+    /**
+     *
+     * @param callback - MUST BE FROM A Upload child.
+     */
+    constructor( server_id: number ){
+
+        this.do_request( server_id );
+
+    }
+
+    private do_request( server_id : number ){
+
+        jQuery.ajax({
+            type: "POST",
+            url: _romhackplaza_script_uploader.submit_url,
+            data: {
+                action: 'set_favorite_server',
+                post_id: I.reserved_post_id,
+                favorite_server: server_id,
+                _wpnonce: _romhackplaza_script_uploader.set_favorite_nonce
+            },
+            success: function(){
+                console.log( "Favorite server set" );
+            }
+        });
+
+    }
+}
\ No newline at end of file
diff --git a/ts/submissions/class-upload.ts b/ts/submissions/class-upload.ts
index dffbd78..c7c9312 100644
--- a/ts/submissions/class-upload.ts
+++ b/ts/submissions/class-upload.ts
@@ -1,5 +1,7 @@
-import {__, FORBIDDEN_CARS, I } from "./globals";
+import {__, FORBIDDEN_CARS, I, API } from "./globals";
 import {DropContainer} from "./class-drop-container";
+import {Reserve_Post_ID} from "./class-reserve-post-id";
+import {Set_Favorite_Server} from "./class-set-favorite-server";
 declare const _romhackplaza_script_uploader: any;
 declare const romhackplaza_modal_submissions: any;
 
@@ -27,6 +29,7 @@ export class Upload {
         }
 
         this.switchDuringUpload();
+        // @ts-ignore
         if( typeof I.drop_container !== 'undefined' && I.drop_container instanceof DropContainer )
             I.drop_container.switch();
 
@@ -36,8 +39,56 @@ export class Upload {
             this.progress_bar = progress.querySelector('.bar') as HTMLElement;
         }
 
-        if( I.reserved_post_id === undefined || I.reserved_post_id === null )
-            console.log( "ok" );
+        if( I.reserved_post_id === undefined || I.reserved_post_id === null ){
+            // Reserve a post ID.
+            new Reserve_Post_ID( this.step1_checkFileExists );
+        } else
+            // Already reserved.
+            this.step1_checkFileExists();
+
+    }
+
+    step1_checkFileExists = () => {
+
+        if( I.favorite_server_set === false ){
+            // @ts-ignore
+            new Set_Favorite_Server( chosenDownloadServerId || 0 );
+            I.favorite_server_set = true;
+        }
+
+        API.check_file_existence( this.file.name ).then( exists => {
+
+            if( !exists ){
+                this.step3_uploadFile();
+                // @ts-ignore
+                document.getElementById( 'cancelButton' )?.style.display = 'block';
+            } else {
+
+                let want_to_delete = window.confirm( __( "File already exists! Proceed with upload and overwrite it ?" ) );
+                if( want_to_delete )
+                    this.step2_deleteFileBeforeUpload();
+
+                else{
+
+                    // @ts-ignore
+                    if( typeof I.drop_container !== 'undefined' && I.drop_container instanceof DropContainer )
+                        I.drop_container.switch();
+                    this.switchDuringUpload();
+
+                }
+
+            }
+
+        } );
+
+    }
+
+    step2_deleteFileBeforeUpload = () => {
+
+
+    }
+
+    step3_uploadFile = () => {
 
     }
 
diff --git a/ts/submissions/class-uploader-data.ts b/ts/submissions/class-uploader-data.ts
index bff8f57..de6023c 100644
--- a/ts/submissions/class-uploader-data.ts
+++ b/ts/submissions/class-uploader-data.ts
@@ -1,21 +1,31 @@
 export class Uploader_Data {
+    [x: string]: any // @ts-ignore
+    ;
 
+    // @ts-ignore
     private data: object;
 
     constructor() {
         this.data = {};
 
         return new Proxy( this, {
-            get: ( target, p ) => {
+            get: ( target, p, receiver ) => {
+                if (p in target) {
+                    return Reflect.get(target, p, receiver);
+                }
                 return target.get( p as string );
             },
-            set: ( target, p, v ) => {
+            set: ( target, p, v , receiver) => {
+                if (p in target) {
+                    return Reflect.set(target, p, v, receiver);
+                }
                 target.set( p as string, v );
                 return true
             }
         });
     }
 
+    // @ts-ignore
     add_field( field_name: string, field_value: any, readonly: boolean ){
 
         // @ts-ignore
@@ -25,6 +35,7 @@ export class Uploader_Data {
 
     }
 
+    // @ts-ignore
     get( field_name : string ): any {
 
         // @ts-ignore
@@ -36,6 +47,7 @@ export class Uploader_Data {
 
     }
 
+    // @ts-ignore
     set( field_name: string, field_value: any ){
 
         // @ts-ignore
diff --git a/ts/submissions/globals.ts b/ts/submissions/globals.ts
index 0bba185..05e9471 100644
--- a/ts/submissions/globals.ts
+++ b/ts/submissions/globals.ts
@@ -1,4 +1,5 @@
 import {Uploader_Data} from "./class-uploader-data";
+import {Server_Api} from "./class-server-api";
 
 export const { __, _x, _n, _nx } = wp.i18n;
 
@@ -8,10 +9,14 @@ export const FORBIDDEN_CARS: Array<string> = [
 
 export const I : Uploader_Data = new Uploader_Data();
 I.add_field( 'is_edit', false, true );
+I.add_field( 'custom_post_type', undefined, true );
 I.add_field( 'reserved_post_id', undefined, false );
 I.add_field( 'during_upload', false, false );
 I.add_field( 'upload', undefined, false );
 I.add_field( 'drop_container', undefined, true );
+I.add_field( 'favorite_server_set', false, false );
+
+export const API : Server_Api = new Server_Api();
 
 export interface Can_Upload_Detail {
     file: File;
diff --git a/ts/submissions/index.ts b/ts/submissions/index.ts
index 8fa9d43..4a76624 100644
--- a/ts/submissions/index.ts
+++ b/ts/submissions/index.ts
@@ -9,7 +9,10 @@ document.addEventListener('DOMContentLoaded', () => {
 
     // Add PostID if edit.
     let url_params = new URLSearchParams(window.location.search);
+    // @ts-ignore
     I.is_edit = url_params.has( 'edit_entry' );
+
+    I.custom_post_type = jQuery( 'input[name="custom_post_type"]').val() || 'unknown';
     I.reserved_post_id = url_params.get('edit_entry' ) || null;
 
     // @ts-ignore