image-file-name-regexps

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

nil

Documentation:
List of regexps matching image-file filenames.
Filenames matching one of these regexps are considered image files,
in addition to those with an extension in `image-file-name-extensions'.

See function `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.