Support uppercase file extentions (#535)

Author: itai-red-id

src/ServiceStack.Text/HttpUtils.cs

@@ -1,4 +1,4 @@
-//Copyright (c) ServiceStack, Inc. All Rights Reserved.
+//Copyright (c) ServiceStack, Inc. All Rights Reserved.
 //License: https://raw.github.com/ServiceStack/ServiceStack/master/license.txt
 
 using System;
@@ -1556,7 +1556,7 @@ public static string GetMimeType(string fileNameOrExt)
             if (string.IsNullOrEmpty(fileNameOrExt))
                 throw new ArgumentNullException(nameof(fileNameOrExt));
 
-            var fileExt = fileNameOrExt.LastRightPart('.');
+            var fileExt = fileNameOrExt.LastRightPart('.').ToLower();
             if (ExtensionMimeTypes.TryGetValue(fileExt, out var mimeType))
             {
                 return mimeType;