Tridion core service How to download binary file of a multimedia component
core_service.ServiceReference1.SessionAwareCoreService2010Client client = new SessionAwareCoreService2010Client();
client.ClientCredentials.Windows.ClientCredential.UserName = "myUserName";
client.ClientCredentials.Windows.ClientCredential.Password = "myPassword"; client.Open();
ComponentData component = (ComponentData)client.TryCheckOut(
                            multimediaComponentURI, new ReadOptions());
BinaryContentData binaryData =   component.BinaryContent;