PHP: hook_field_widget_form_alter() is what ImageInfo Cache uses to get the file right after it has been uploaded on the PHP side. I use this to add a #process callback which then adds a callback to the #submit button. All of this to generate image styles in a background process right after the image was uploaded.
If you want a more barbaric way file_get_contents("php://input") & $_POST will give you access to everything needed. This is not advised & a bad idea. More info about the PHP wrapper.