mu: use '.msgpart' as extension

We were using '.part', but some desktop software interprets that as a
partial download.

lib/mu-msg-part.c

@@ -271,7 +271,7 @@ guess_file_name (GMimeObject *mobj, unsigned index)
 		return g_strdup_printf ("vcal-%u.vcs", index);
 
 	/* fallback */
-	return g_strdup_printf ("%u.part", index);
+	return g_strdup_printf ("%u.msgpart", index);
 }