Add IsFileNameValid in ContentProxy.

Source/Editor/Content/Proxy/ContentProxy.cs

@@ -73,6 +73,16 @@ namespace FlaxEditor.Content
             throw new NotImplementedException();
         }
 
+        /// <summary>
+        /// Determines whether the specified filename is valid for this proxy.
+        /// </summary>
+        /// <param name="filename">The filename.</param>
+        /// <returns><c>true</c> if the filename is valid, otherwise <c>false</c>.</returns>
+        public virtual bool IsFileNameValid(string filename)
+        {
+            return true;
+        }
+        
         /// <summary>
         /// Determines whether this proxy can create items in the specified target location.
         /// </summary>