Further explain the filename character blacklist

src/addon/client.cpp

@@ -156,7 +156,11 @@ bool addons_client::upload_addon(const std::string& id, std::string& response_me
 	if(!check_names_legal(addon_data, &badnames)){
 		this->last_error_ =
 			vgettext("The add-on <i>$addon_title</i> has an invalid file or directory "
-				"name and cannot be published.", i18n_symbols);
+				"name and cannot be published. "
+
+				"File or directory names may not contain whitespace, control characters or any of the following characters: '&quot; * / : &lt; &gt; ? \\ | ~'. "
+				"It also may not contain '..' end with '.' or be longer than 255 characters."
+				, i18n_symbols);
 		this->last_error_data_ = font::escape_text(utils::join(badnames, "\n"));
 		return false;
 	}

src/campaign_server/campaign_server.cpp

@@ -690,7 +690,8 @@ void server::handle_upload(const server::request& req)
 		LOG_CS << "Upload aborted - invalid file names in add-on data (" << badnames.size() << " entries).\n";
 		send_error(
 			"Add-on rejected: The add-on contains files or directories with illegal names. "
-			"File or directory names may not contain whitespace or any of the following characters: '/ \\ : ~'",
+			"File or directory names may not contain whitespace, control characters or any of the following characters: '\" * / : < > ? \\ | ~'. "
+			"It also may not contain '..' end with '.' or be longer than 255 characters.",
 			filelist, req.sock);
 	} else if(!check_case_insensitive_duplicates(data)) {
 		LOG_CS << "Upload aborted - case conflict in add-on data.\n";