把mp4和xlsx混合上传时, 会有文件missing

例如4个mp4和4个xlsx上传, 会不见了好大一部xlsx和小部分mp4文件
![image](https://cloud.githubusercontent.com/assets/26402108/24802780/c7e1ca4c-1bdb-11e7-8eb1-2ae33f100d46.png)
![image](https://cloud.githubusercontent.com/assets/26402108/24802837/eeb4db8c-1bdb-11e7-8ff8-f9ed935c4864.png)
