-node-id reversed entity-number (list of integers)
-point-min beginning point of region in raw-buffer
-point-max end point of region in raw-buffer
-type media-type (symbol)
-subtype media-subtype (symbol)
-type/subtype media-type/subtype (string or nil)
-parameters parameter of Content-Type field (association list)
-encoding Content-Transfer-Encoding (string or nil)
-children entities included in this entity (list of content-infos)
+buffer buffer includes this entity (buffer).
+node-id node-id (list of integers)
+header-start minimum point of header in raw-buffer
+header-end maximum point of header in raw-buffer
+body-start minimum point of body in raw-buffer
+body-end maximum point of body in raw-buffer
+content-type content-type (content-type)
+content-disposition content-disposition (content-disposition)
+encoding Content-Transfer-Encoding (string or nil)
+children entities included in this entity (list of entity)