Getting an image from Wikipedia API. MD5 of the image name not same as wikipedia md5 code.

Hello, I am trying to get images from wikipedia API. API only gives name of the files. According to here wikipedia encodes filename with md5 and puts it under:

http://upload.wikimedia.org/wikipedia/commons/FIRST_CHAR_MD5/FIRST_TWO_CHAR_MD5/FILE_NAME

But when i try to encode filenames with c#'s md5 algorithm i get different md5 hash code. How can i get same hashcode?

         System.Security.Cryptography.MD5 md5 = System.Security.Cryptography.MD5.Create();
         md5.Initialize();
         byte[] buffer = System.Text.Encoding.ASCII.GetBytes(filename);
         byte[] bufferEncrypt = md5.ComputeHash(buffer, 0, buffer.Length);
         string pwdX = System.Convert.ToBase64String(bufferEncrypt);
         md5.Clear();
         byte[] bytes = System.Convert.FromBase64String(pwdX); 
         StringBuilder sb = new StringBuilder(); 
         foreach (byte b in bytes) 
              sb.Append(b.ToString("x2")); 
         print (sb.ToString()); 

EDIT: Seems like the problem is with the specific characters like "% ü .....".