Added IPath

ProcessFiles/Interfaces/IFileSystem.cs

@@ -4,5 +4,6 @@
     {
         public IFile File { get; }
         public IDirectory Directory { get; }
+        public IPath Path { get; }
     }
 }

ProcessFiles/Interfaces/IPath.cs

@@ -0,0 +1,7 @@
+namespace ProcessFiles.Interfaces
+{
+    public interface IPath
+    {
+        public string? GetExtension(string? path);
+    }
+}

ProcessFiles/Models/DefaultFileSystem.cs

@@ -9,5 +9,8 @@ namespace ProcessFiles.Models
 
         private readonly IDirectory directory = new DefaultDirectory();
         public IDirectory Directory => directory;
+
+        private readonly IPath path = new DefaultPath();
+        public IPath Path => path;
     }
 }

ProcessFiles/Models/DefaultPath.cs

@@ -0,0 +1,10 @@
+using ProcessFiles.Interfaces;
+using System.IO;
+
+namespace ProcessFiles.Models
+{
+    public class DefaultPath : IPath
+    {
+        public string? GetExtension(string? path) => Path.GetExtension(path);
+    }
+}

ProcessFiles/ProcessFiles.cs

@@ -32,7 +32,7 @@ namespace ProcessFiles
 
         private string? GetExtension(string path)
         {
-            var extension = Path.GetExtension(path).TrimStart('.');
+            var extension = fileSystem.Path.GetExtension(path)?.TrimStart('.');
             if (!string.IsNullOrWhiteSpace(extension)) return extension;
             
             errors.Add($"Can't establish extension of {path}!");