File Upload & Download using ASP.net 3.5 & MVC2.0

I am trying upload file using the below code .But in the controller HTTPPostedFile is coming  as null ? Any solution for this .

<% using (Ajax.BeginForm("Upload", "Attachment", new AjaxOptions { OnSuccess = "ProcessAttachmentUpload", LoadingElementId = "imgAjaxLoader" }, new { enctype = "multipart/form-data" }))

       { %>
        <p>
            <%= Html.HiddenFor(model => model.RequestID)%>
        </p>
             
        <div id="QuestionCreateValidationMessage" class="validation-summary-errors"></div>        
        <table class="Main">          
            <tr>
               <td>    
                        <%= Html.LabelFor(model => model.Name)%>
                        <%= Html.TextBoxFor(model => model.Name, new { @class = "ui-widget-content" })%>
               </td>
            </tr>      
            <tr>
                <td>
                    <input type="file"  name="FileUpload" id="FileUpload" value="Uploadfile"/>
                   
                </td>
            </tr>      
        </table>        
        <p>
            <input type="submit" class="ui-button ui-state-default" id="btnUpload" value="Upload" />
        </p>        
    <% } %>