"nifti",
"niftiImage" or "internalImage", then the result is always
TRUE. Likewise if it has an internal image pointer (although in that
case it should also be of class "niftiImage"). If it has no
dim attribute, or looks like an affine matrix, then the result is
FALSE. Otherwise the value of the unsure argument is returned.isImage(object, unsure = NA)object is an image.