mu: special-case text/calendar parts

Set the name for (otherwise unnamed) mime-parts of type text/calendar to
"vcal-<n>.vcs"; this allows for easier processing with external tools.

lib/mu-msg-part.c

@@ -251,6 +251,30 @@ cleanup_filename (char *fname)
 }
 
 
+/*
+ * when a part doesn't have a filename, it can be useful to 'guess' one based on
+ * its mime-type, which allows other tools to handle them correctly, e.g. from
+ * mu4e.
+ *
+ * For now, we only handle calendar invitations in that way, but others may
+ * follow.
+ */
+static char*
+guess_file_name (GMimeObject *mobj, unsigned index)
+{
+	GMimeContentType *ctype;
+
+	ctype = g_mime_object_get_content_type (mobj);
+
+	/* special case for calendars; map to '.vcs' */
+	if (g_mime_content_type_is_type (ctype, "text", "calendar"))
+		return g_strdup_printf ("vcal-%u.vcs", index);
+
+	/* fallback */
+	return g_strdup_printf ("%u.part", index);
+}
+
+
 static char*
 mime_part_get_filename (GMimeObject *mobj, unsigned index,
 			gboolean construct_if_needed)
@@ -279,10 +303,11 @@ mime_part_get_filename (GMimeObject *mobj, unsigned index,
 	}
 
 	if (!fname)
-		fname =	g_strdup_printf ("%u.part", index);
+		fname = guess_file_name (mobj, index);
 	
 	/* remove slashes, spaces, colons... */
 	cleanup_filename (fname);
+
 	return fname;
 }