code: public static String getByFilename(String fileName, String defaultType) {
          String extn = getExtension(fileName);
          if (extn != null) {
              return get(extn, defaultType);
          } else {
              // no extension, no content type
              return null;
          }
      }
docs: <p>getByFilename.</p>
  
  @param fileName    the filename
  @param defaultType default Type
  @return the content type associated with <code>extension</code> of the
  given filename or if no associate is found, returns
  <code>defaultType</code>