mshow: show last part of multipart/mixed when none matched

This is in conformance with RFC 1341.

man/mshow.1

@@ -56,7 +56,7 @@ in the order given,
 when rendering
 .Sq Li "multipart/alternative"
 parts.
-If no MIME type matches, the first MIME part will be rendered.
+If no MIME type matches, the last MIME part will be rendered.
 .Pp
 When
 .Ar mimetypes

mshow.c

@@ -336,7 +336,7 @@ nofilter:
 static void
 choose_alternative(struct message *msg, int depth)
 {
-	int n = 1;
+	int n = 0;
 	int m = 0;
 	char *p = Aflag + strlen(Aflag);
 
@@ -363,6 +363,13 @@ choose_alternative(struct message *msg, int depth)
 	}
 	blaze822_free(imsg);
 
+	if (n == 0) {
+		// No part matched, use last part as per RFC1341 7.2.3
+		// "In general, choosing the best type means displaying
+		// only the LAST part that can be displayed."
+		n = m;
+	}
+
 	imsg = 0;
 	while (blaze822_multipart(msg, &imsg))
 		if (--n == 0)