laravel can not get image's extension

I am using Laravel 5.3,

I crop an image via javascript on the front end and upload it to back end via ajax,

The headers is like this:

And dd() the image:

    public function changeAvatar(Request $request)
    {
    
        $file = $request->file('croppedImage');
        dd($file);
    
    }

The result is like this:

But,dd() the extension of the image:

    public function changeAvatar(Request $request)
    {
        $file = $request->file('croppedImage');
        $extension = $file->getClientOriginalExtension();
        dd($extension);
    }

The result is ''.

Why is it ?