code: public static String typeOfSuffix(String fileExtension) {
          MimeType mimeType = indexByFileExtension.get(fileExtension);
          return null == mimeType ? fileExtension : mimeType.type;
      }
docs: Return a content type string corresponding to a given file extension suffix.
  
  If there is no MimeType corresponding to the file extension, then returns the file
  extension string directly.
  
  @param fileExtension
  file extension suffix
  @return
  A content type string corresponding to the file extension suffix
  or the file extension suffix itself if no corresponding mimetype found.