VerifyError: Error #1014: The class was not found flash.filesystem::File

I have a project in adobe air that it is assumed that in the i can work with files using the File class but when I click on the button I get the following error:

VerifyError: Error #1014: The class was not found  flash.filesystem::File.

at cubo_fla::MainTimeline/btnFrontClick()

I am playing the swf with flash player 10.

The following code is what has the button to be pressed.

function btnFrontClick(e:MouseEvent):void

     {

        var archivo:File=File.applicationDirectory;

        var target=new File(archivo.nativePath + "\\TempImagesZoom");

        archivo=archivo.resolvePath("image1.jpg");

        var targetParent:File=target.parent;

        targetParent.createDirectory();

        target=target.resolvePath("small.jpg");

        archivo.copyTo(target,true);

        archivo=File.applicationDirectory;

        target=new File(archivo.nativePath + "\\TempImagesZoom\\big.jpg");

        archivo=archivo.resolvePath("image1Big.jpg");

        archivo.copyTo(target,true);

        trace("Archivocopiado.");

        cube.visible=false;

        btnFront.visible=false;

         btnLeft.visible=false;

         btnRight.visible=false;

         btnBack.visible=false;

         btnUp.visible=false;

         btnDown.visible=false;

         btnFront.enabled=true;

        var i=new Loader();

        this.addChild(i);

        i.load(new URLRequest("lupa.swf"));

     }

Thanks.