FilesInput: Adding missing typing for "startingProcessingFilesCallback"

src/Tools/babylon.filesInput.ts

@@ -19,7 +19,7 @@
         private _filesToLoad: File[];
 
         constructor(engine: Engine, scene: Scene, sceneLoadedCallback: (sceneFile: File, scene: Scene) => void, progressCallback: (progress: SceneLoaderProgressEvent) => void, additionalRenderLoopLogicCallback: () => void, 
-            textureLoadingCallback: (remaining: number) => void, startingProcessingFilesCallback: () => void, onReloadCallback: (sceneFile: File) => void, errorCallback: (sceneFile: File, scene: Scene, message: string) => void) {
+            textureLoadingCallback: (remaining: number) => void, startingProcessingFilesCallback: (files?: File[]) => void, onReloadCallback: (sceneFile: File) => void, errorCallback: (sceneFile: File, scene: Scene, message: string) => void) {
             this._engine = engine;
             this._currentScene = scene;