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,
-were abolished.
+'mime-view-image-converter-alist were abolished.
Preview-situation is generated from entity information, running
environment and 'mime-preview-condition. These elements are checked
specify body-filter function. If 'body-presentation-method is
function, it is called to generate presentation of entity body.
+Body-filter function 'mime-view-filter-for-image refers 'image-format.
+
Setting for message/partial button is specified by
'mime-preview-condition instead of hard-coding.