// IService.cs
[ServiceContract]
public interface IService
{
    [OperationContract]
    void TestFile(byte[] file);
}

// Service.svc.cs
public class Service : IService
{
    public void TestFile(byte[] file)
    {
        
    }
}

// web.config сервиса
<system.serviceModel>
    
    <services>
        <service ... >
            ...
            <endpoint address="/basicSoap" binding="basicHttpBinding" contract="IService">
            </endpoint>
            ...
        </service>
    </services>
    ...
    <bindings>
        <basicHttpBinding>
            <binding name="ff" 
                transferMode="Streamed"
                messageEncoding="Mtom"
                maxReceivedMessageSize="10067108864">
            </binding>
        </basicHttpBinding>
    </bindings>

if (FileUpload1.HasFile)
{
    using (localhost.Service srv = new localhost.Service())
    {
        srv.TestFile(FileUpload1.FileBytes);
    }
}