Only non-null values are put into Map<String,String> mimeTypeMap. It means, we can replace containsKey+put calls with single putIfAbsent call. It makes code a bit cleaner and faster.

 private void putIfAbsent(String key, String value) {
     if (key != null && !key.isEmpty() &&
         value != null && !value.isEmpty() &&
         !mimeTypeMap.containsKey(key))
     {
         mimeTypeMap.put(key, value);
     }
 }