The underlying connection was closed: An unexpected error occurred on a receive (After file upload completed) - asp.net-mvc

Question

In Asp.Net MVC 5
I use this code in a class to upload my file in FTP Server.
FtpWebRequest ftpRequest = (FtpWebRequest)WebRequest.Create(ftpURL);
ftpRequest.Method = WebRequestMethods.Ftp.UploadFile;
ftpRequest.Credentials = new NetworkCredential(UserName, Password);
ftpRequest.Timeout = -1;
Stream requestStream = ftpRequest.GetRequestStream();
long fileSize = 0;
Byte[] buffer = new Byte[buferSize];
int bytesRead = file.Read(buffer, 0, buferSize);
fileSize += bytesRead;
while (bytesRead > 0)
{
requestStream.Write(buffer, 0, bytesRead);
bytesRead = file.Read(buffer, 0, buferSize);
fileSize += bytesRead;
}
requestStream.Close();
return fileSize;
and I have this config in web.config because files are too big:
<location path="Programs/Upload">
<system.web>
<httpRuntime executionTimeout="86400" maxRequestLength="2097152" />
</system.web>
<system.webServer>
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="2147483648"/>
</requestFiltering>
</security>
</system.webServer>
When I try to upload small files (in release mode with debug = 'false') everything work well. but when file size become big I face with this error (after file completely uploaded):
**The underlying connection was closed: An unexpected error occurred on a receive**
The file is completely uploaded and is available in FTP Server!