+*** mime-preview-condition and preview-situation
+
+ Conditions about preview generation are unified to
+'mime-preview-condition. Namely other variables, such as
+'mime-view-visible-media-type-list, 'mime-view-content-filter-alist,
+'mime-view-image-converter-alist were abolished.
+
+ Preview-situation is generated from entity information, running
+environment and 'mime-preview-condition. These elements are checked
+to match with each other. (it is similar to acting-situation)
+
+ Format of preview-situation is association-list. Following key is
+pre-defined:
+
+ 'type media-type
+ 'subtype media-subtype
+ 'encoding content-transfer-encoding
+ 'major-mode major-mode of MUA
+ <STRING> attribute of Content-Type field.
+ 'body-presentation-method body-presentation-method
+
+If 'body-presentation-method is 'with-filter, 'body-filter is used to
+specify body-filter function. If 'body-presentation-method is
+function, it is called to generate presentation of entity body.