Function: image-type

Determine and return image type.
SOURCE is an image file name or image data.
Optional TYPE is a symbol describing the image type. If TYPE is omitted
or nil, try to determine the image type from its first few bytes
of image data. If that doesn't work, and SOURCE is a file name,
use its file extension as image type.
Optional DATA-P non-nil means SOURCE is a string containing image data.