diff --git a/src/mu-msg.c b/src/mu-msg.c
index 3833a117..0531ba3a 100644
--- a/src/mu-msg.c
+++ b/src/mu-msg.c
@@ -107,6 +107,16 @@ mu_msg_new_from_file (const char *path, const char *mdir, GError **err)
 }
 
 
+void
+mu_msg_close_file_backend (MuMsg *msg)
+{
+	g_return_if_fail (msg);
+
+	mu_msg_file_destroy (msg->_file);
+	msg->_file = NULL;
+}
+
+
 MuMsg*
 mu_msg_new_from_doc (XapianDocument *doc, GError **err)
 {
@@ -310,6 +320,7 @@ get_num_field (MuMsg *self, MuMsgFieldId mfid)
 {
 	guint64 val;
 
+	/* first try the cache */
 	if (mu_msg_cache_cached (self->_cache, mfid))
 		return mu_msg_cache_num (self->_cache, mfid);
 
diff --git a/src/mu-msg.h b/src/mu-msg.h
index 1b672b23..f8edebb6 100644
--- a/src/mu-msg.h
+++ b/src/mu-msg.h
@@ -69,6 +69,18 @@ MuMsg *mu_msg_new_from_doc (XapianDocument* doc, GError **err)
 	G_GNUC_MALLOC G_GNUC_WARN_UNUSED_RESULT;
 
 
+/**
+ * close the file-backend, if any; this function is for the use case
+ * where you have a large amount of messages where you need some
+ * file-backed field (body or attachments). If you don't close the
+ * file-backend after retrieving the desired field, you'd quickly run
+ * out of file descriptors. If this message does not have a
+ * file-backend, do nothing.
+ *
+ * @param msg a message object
+ */
+void mu_msg_close_file_backend (MuMsg *msg);
+
 /**
  * increase the reference count for this message
  *