Support "extra" folder for extras content (#11249)

Emby.Naming/Common/NamingOptions.cs

@@ -537,6 +537,12 @@ namespace Emby.Naming.Common
                     "extras",
                     MediaType.Video),
 
+                new ExtraRule(
+                    ExtraType.Unknown,
+                    ExtraRuleType.DirectoryName,
+                    "extra",
+                    MediaType.Video),
+
                 new ExtraRule(
                     ExtraType.Unknown,
                     ExtraRuleType.DirectoryName,