diff --git a/Creek.FileRepository/Helpers/StringHelper.cs b/Creek.FileRepository/Helpers/StringHelper.cs
index fa19330..81e865b 100644
--- a/Creek.FileRepository/Helpers/StringHelper.cs
+++ b/Creek.FileRepository/Helpers/StringHelper.cs
@@ -1,20 +1,21 @@
 using System;
 using System.Collections.Generic;
 using System.Text;
+using System.Text.RegularExpressions;
 
 namespace Creek.FileRepository.Helpers
 {
     /// <summary>
     /// A helper class for string methods.
     /// </summary>
-    internal class StringHelper
+    internal static class StringHelper
     {
         /// <summary>
         /// Checks if a filename contains invalid characters.
         /// </summary>
         /// <param name="filename">The filename to be checked.</param>
         /// <returns>Returns true if the filename contains invalid characters.</returns>
-        public static bool IsInvalidFileName(string filename)
+        public static bool IsInvalidFileName(this string filename)
         {
             char[] invalidCharacters = new char[] { '<', '>', ':', '\'', '/', '\\', '|', '?', '*' };
 
diff --git a/Creek.FileRepository/Repositories/SftpRepository.cs b/Creek.FileRepository/Repositories/SftpRepository.cs
index d7557d2..ce6ad90 100644
--- a/Creek.FileRepository/Repositories/SftpRepository.cs
+++ b/Creek.FileRepository/Repositories/SftpRepository.cs
@@ -42,7 +42,7 @@ namespace Creek.FileRepository.Repositories
         /// <returns>Returns the filename.</returns>
         public async Task<string> CreateFileAsync(RepoFile file)
         {
-            if (StringHelper.IsInvalidFileName(file.Filename))
+            if (file.Filename.IsInvalidFileName())
             {
                 throw new ArgumentException($"The filename for'{nameof(file)}' contains invalid characters for a filename.", nameof(file));
             }
@@ -64,7 +64,7 @@ namespace Creek.FileRepository.Repositories
             {
                 throw new ArgumentException($"'{nameof(filename)}' cannot be null or empty.", nameof(filename));
             }
-            else if (StringHelper.IsInvalidFileName(filename))
+            else if (filename.IsInvalidFileName())
             {
                 throw new ArgumentException($"'{nameof(filename)}' contains invalid characters for a filename.", nameof(filename));
             }
@@ -102,7 +102,7 @@ namespace Creek.FileRepository.Repositories
             {
                 throw new ArgumentException($"'{nameof(filename)}' cannot be null or empty.", nameof(filename));
             }
-            else if (StringHelper.IsInvalidFileName(filename))
+            else if (filename.IsInvalidFileName())
             {
                 throw new ArgumentException($"'{nameof(filename)}' contains invalid characters for a filename.", nameof(filename));
             }
@@ -140,7 +140,7 @@ namespace Creek.FileRepository.Repositories
         /// <returns>Returns true if update was successful.</returns>
         public async Task<bool> UpdateFileAsync(RepoFile file)
         {
-            if (StringHelper.IsInvalidFileName(file.Filename))
+            if (file.Filename.IsInvalidFileName())
             {
                 throw new ArgumentException($"The filename for'{nameof(file)}' contains invalid characters for a filename.", nameof(file));
             }