#2035 - Centralize definitions of how thumb/resize filenames are generated
- added ALBUM_IMAGE_NAME and EXTENSION_CHANGES to Item_Model.
- added _convert_name_by_size() and _get_mime_type_by_size() to Item_Model to process them.
- added thumb_mime_type() and resize_mime_type() to Item_Model.
- revised data_rest to use new mime functions.
- revised file_proxy to use new mime functions.
@todo: revise/move item::find_by_path.