Reproduce:
```
var sftpClient = new SftpClient( server, port, user,
new PrivateKeyFile( new MemoryStream( Encoding.ASCII.GetBytes( privateKey ) ) ) );
// 4* 1024 - data will be corrupted, 1*1024 - data will be transferred OK
var dataToUpload = new string( '0', 4 * 1024 ) + "Z";
sftpClient.Connect();
sftpClient.WriteAllText( "incoming/text.txt", dataToUpload, Encoding.ASCII );
```
```
var sftpClient = new SftpClient( server, port, user,
new PrivateKeyFile( new MemoryStream( Encoding.ASCII.GetBytes( privateKey ) ) ) );
// 4* 1024 - data will be corrupted, 1*1024 - data will be transferred OK
var dataToUpload = new string( '0', 4 * 1024 ) + "Z";
sftpClient.Connect();
sftpClient.WriteAllText( "incoming/text.txt", dataToUpload, Encoding.ASCII );
```