* @param $width int Max width of thumbnail in pixels. (if null, use common_config values)
* @param $height int Max height of thumbnail in pixels. (if null, square-crop to $width)
* @param $crop bool Crop to the max-values' aspect ratio
+ * @param $force_still bool Don't allow fallback to showing original (such as animated GIF)
+ * @param $upscale mixed Whether or not to scale smaller images up to larger thumbnail sizes. (null = site default)
*
* @return File_thumbnail
*