server/message-part: restore forwarding attachments

lib/message/mu-message-part.cc

@@ -95,6 +95,15 @@ MessagePart::mime_type() const noexcept
 		return Nothing;
 }
 
+Option<std::string>
+MessagePart::content_description() const noexcept
+{
+	if (!mime_object().is_part())
+		return Nothing;
+	else
+		return MimePart{mime_object()}.content_description();
+}
+
 size_t
 MessagePart::size() const noexcept
 {

lib/message/mu-message-part.hh

@@ -87,6 +87,14 @@ public:
 	 */
 	Option<std::string> mime_type() const noexcept;
 
+
+	/**
+	 * Get the content description for this part, or Nothing
+	 *
+	 * @return the content description
+	 */
+	Option<std::string> content_description() const noexcept;
+
 	/**
 	 * Get the length of the (unencoded) MIME-part.
 	 *