Better support for image files:
* [FILE_EXT,SC] Removed is_image_file/1, iamge_extension/1. See IMAGE_EXT.
* [IMAGE_EXT,NEW] Added image_file/1, image_file_extension/1, image_url/1.
* [URI_EXT,SC] Removed is_iamge_url/1. See IMAGE_EXT.