code: public Response changeMime(String bucket, String key, String mime)
              throws QiniuException {
          String resource = encodedEntry(bucket, key);
          String encodedMime = UrlSafeBase64.encodeToString(mime);
          String path = String.format("/chgm/%s/mime/%s", resource, encodedMime);
          return rsPost(bucket, path, null);
      }
docs: 修改文件的MimeType
  
  @param bucket 空间名称
  @param key    文件名称
  @param mime   文件的新MimeType
  @throws QiniuException
  @link http://developer.qiniu.com/kodo/api/chgm