code: public static boolean isExtension(String filename, Collection extensions) {
          if (filename == null) {
              return false;
          }
          if ((extensions == null) || extensions.isEmpty()) {
              return (indexOfExtension(filename) == -1);
          }
          String fileExt = getExtension(filename);
          for (Iterator it = extensions.iterator(); it.hasNext();) {
              if (fileExt.equals(it.next())) {
                  return true;
              }
          }
          return false;
      }
docs: Checks whether the extension of the filename is one of those specified.
  <p>
  This method obtains the extension as the textual part of the filename
  after the last dot. There must be no directory separator after the dot.
  The extension check is case-sensitive on all platforms.
  
  @param filename  the filename to query, null returns false
  @param extensions  the extensions to check for, null checks for no extension
  @return true if the filename is one of the extensions