Update HttpUtils.cs

Author: mythz

src/ServiceStack.Text/HttpUtils.cs

@@ -1363,9 +1363,7 @@ public static string GetMimeType(string fileNameOrExt)
             if (string.IsNullOrEmpty(fileNameOrExt))
                 throw new ArgumentNullException(nameof(fileNameOrExt));
 
-            var parts = fileNameOrExt.Split('.');
-            var fileExt = parts[parts.Length - 1];
-
+            var fileExt = fileNameOrExt.LastRightPart('.');
             if (ExtensionMimeTypes.TryGetValue(fileExt, out var mimeType))
             {
                 return mimeType;