用httpput上传的文件在开头和结尾都有额外的数据

我在IIS中创建了一个虚拟目录,并在程序中执行以下代码:

System.Net.WebClient webClient = new System.Net.WebClient();
webClient.Credentials = System.Net.CredentialCache.DefaultCredentials;
webClient.UploadFile( "http://localhost/Exchange/file.jpg", "PUT", localPathToMyFile );

-----------------------8ccfabf2a995855
Content-Disposition: form-data; name="file"; filename="file.jpg"
Content-Type: application/octet-stream
//file contents goes here
-----------------------8ccfabf2a995855

这些额外的数据从何而来?我该怎么做才能使它不出现,但文件是按原样上传的?