code: public Bitmap loadFile(String fileName, String tmpDirectory, ImageCallBack imageCallBack,
                             boolean canRemove) {
  
          return loadFile(fileName, tmpDirectory, imageCallBack, canRemove, -1, -1);
      }
docs: Load Steps:<br/>
  1. load from caches. <br/>
  2. if file not exist in caches, then load it from local storage.<br/>
  3. if local storage not exist, then load it from temporary directory.<br/>
  4. if file not exist in temporary directory. then download it.<br/>
  5. after download, then callback will be added.<br/>
  
  @param fileName      file name , local or URL 可以是带路径名, 也可以不带路径名. 当带路径名时, 先去查找指定目录..
  然后再找傳入的臨時目錄.
  @param tmpDirectory  临时目录, 下载之后的保存目录. 默认路径: mnt/sdcard/cache
  @param imageCallBack 下载完成时的毁掉
  @param canRemove     是否可以移除. 当等待的个数超过最大下载数目时, 当前条目是否可以清除.
  @return the bitmap loaded. if from network, this will return null,
  return the result from callback