image-format-suffixes

image-format-suffixes is a variable defined in `image.el'.
Its value is

((image/x-icon "ico"))

Documentation:
An alist associating image types with file name suffixes.
This is used as a hint by the ImageMagick library when detecting
the type of image data (that does not have an associated file name).
Each element has the form (MIME-CONTENT-TYPE EXTENSION).
If `create-image' is called with a :format attribute whose value
equals a content-type found in this list, the ImageMagick library is
told that the data would have the associated suffix if saved to a file.