+Views a non-text MIME attachment. This hook allows you to run
+third-party programs for attachments that require such a thing (e.g.
+images). To instead display a text version of the attachment directly in
+Sup, use the mime-decode hook instead.
+
+Note that by default (at least on systems that have a run-mailcap command),
+Sup uses the default mailcap handler for the attachment's MIME type. If
+you want a particular behavior to be global, you may wish to change your
+mailcap instead.
+