image-file-name-extensions

image-file-name-extensions is a variable defined in `image-file.el'.
Its value is

("png" "jpeg" "jpg" "gif" "tiff" "tif" "xbm" "xpm" "pbm" "pgm" "ppm" "pnm" "svg")

Documentation:
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.

You can customize this variable.