code: public static void addFileOpener(ServletContext servletContext, FileOpener fileOpener, String ... extensions) {
  		synchronized(fileOpenersLock) {
  			@SuppressWarnings("unchecked")
  			Map<String,FileOpener> fileOpeners = (Map<String,FileOpener>)servletContext.getAttribute(FILE_OPENERS_REQUEST_ATTRIBUTE_NAME);
  			if(fileOpeners == null) {
  				fileOpeners = new HashMap<>();
  				servletContext.setAttribute(FILE_OPENERS_REQUEST_ATTRIBUTE_NAME, fileOpeners);
  			}
  			for(String extension : extensions) {
  				if(fileOpeners.containsKey(extension)) throw new IllegalStateException("File opener already registered: " + extension);
  				fileOpeners.put(extension, fileOpener);
  			}
  		}
  	}
docs: Registers a file opener.
  
  @param  extensions  The simple extensions, in lowercase, not including the dot, such as "dia"