上传大文件时报错

Question

一直使用七牛提供的c#SDK在做附件上传，最近上传几个大文件发现传不上去，两个小于50M的文件能传，一个78M一个80M的都传不上去，抓了一下异常得到下面结果，请问是怎样一个情况：
ret.StatusCode:BadRequest
ret.Responce:
ret.Exception:
System.Net.WebException: 请求被中止: 请求已被取消。
在 System.Net.ConnectStream.InternalWrite(Boolean async, Byte[] buffer, Int32 offset, Int32 size, AsyncCallback callback, Object state)
在 System.Net.ConnectStream.Write(Byte[] buffer, Int32 offset, Int32 size)
在 Qiniu.IO.MultiPart.MultiPost(String url, NameValueCollection formData, Stream inputStream)
在 Qiniu.IO.IOClient.Put(String upToken, String key, Stream putStream, PutExtra extra)

代码如下：

PutPolicy policy = new PutPolicy(Bucket);
string upToken = policy.Token();
PutExtra extra = new PutExtra();
IOClient client = new IOClient();
PutRet ret = client.Put(upToken, key, fs, extra);
if (ret.OK)
{
    return true;
}
else
{
    throw new Exception("Exception: ret.Responce:" + ret.Response+ " ret.Exception:" + ret.Exception + " ret.StatusCode:" + ret.StatusCode);
}