standardize knowledge of supported image extensions

MediaBrowser.Controller/Entities/BaseItem.cs

@@ -42,6 +42,11 @@ namespace MediaBrowser.Controller.Entities
             LocalTrailerIds = new List<Guid>();
         }
 
+        /// <summary>
+        /// The supported image extensions
+        /// </summary>
+        public static readonly string[] SupportedImageExtensions = new[] { ".png", ".jpg", ".jpeg" };
+        
         /// <summary>
         /// The trailer folder name
         /// </summary>