Javascript FileReader有多个文件，如何迭代文件名？

I'm trying to write a multi upload image logic. This is the code i use:

function readURL(input,index) {

                    for (var z = 0; z < input.files.length; z++) {

                        var FileZ=input.files[z];


                        if (input.files && input.files[z]) {
                            var reader = new FileReader();  
                            reader.onload = function (e) {
                                var i = new Image();
                                i.src=e.target.result;
                                i.onload = function(){
                                    //ajax crop to 500x350
                                    var arrExt = FileZ.name.split('.');
                                    if(i.width>=500 && i.height>=350) {
                                        $.ajax({
                                            url: 'ajax/cropImage.php',
                                            type: 'POST',
                                            data: 'base64='+e.target.result+"&ext="+arrExt[(arrExt.length-1)],
                                            success: function(data) {

                                                //here i insert the image in html.
                                            }
                                        });

                                    } else {
                                        alert("Image too big");
                                    }
                                };
                            }                   
                            reader.readAsDataURL(input.files[z]);
                        }
                    }

                }

This code works correctly, i can upload and see after ajaxCrop.php the resulting images. The problem comes when i try to upload image with different extensions. If i try to upload one jpg and one png, only one of this photo will be cropped. The problem is in

var arrExt = FileZ.name.split('.');

Here the file name is always the same, at z=0 name='image.jpg' and at z=1 name='image.jpg' (the name should be 'image.png'). So i get an error when i try to crop a 'false' jpg... How i can get the correct 'name at index'?