image_file_delete() should not call image_path_flush() except for imagefields

image_file_delete() implements hook_file_delete(), which means it's called for every file deletion. However, not every file deletion is an image. It's currently calling the very expensive image_path_flush() anyway for every deletion.

It should somehow try to figure out whether this is an image (more specifically whether this is an imagefield).

/**
 * Implements hook_file_delete().
 */
function image_file_delete($file) {
  // Delete any image derivatives of this image.
  image_path_flush($file->uri);
}