A list of image-file filename extensions.
Filenames having one of these extensions are considered image files,
in addition to those matching `image-file-name-regexps'.
See `auto-image-file-mode'; if `auto-image-file-mode' is enabled,
setting this variable directly does not take effect unless
`auto-image-file-mode' is re-enabled; this happens automatically when
the variable is set using M-x customize.