Mvc image uploder not working

if (itm.AttachmentModel.AttachmentFile != null)
               {
                   itm.AttachmentModel.AttachmentURL = string.Format("{0}.{1}", Guid.NewGuid().ToString(), itm.AttachmentModel.AttachmentFile.FileName.Split('.')[1]);
                   if (itm.AttachmentModel.AttachmentFile != null)
                   {

                       var path = Path.Combine(Server.MapPath("~/App_Data/Attachments/"), itm.AttachmentModel.AttachmentURL);
                       itm.AttachmentModel.AttachmentFile.SaveAs(path);
                   }
               }

<input type="hidden" value="" name="..."><input type="file" name="AttachmentFile">

public HttpPostedFileBase AttachmentFile { get; set; }
    public string AttachmentURL
    {
        get
        {
            return attachment;
        }

        set
        {
            attachment = value;
        }
    }

    public string Attachment
    {
        get
        {
            return attachment;
        }

        set
        {
            attachment = value;
        }
    }