throwing an exception when trying to attach a document to a list item

[JimmyRopes]

I am throwing a System.ArgumentException when trying to attach a document to a list item. This is the code I am using. I had log statements (since removed to eliminate extraneous code) that showed me that I was getting to the

Item.Attachments.AddNow(FileName, binFile);

statement which was throwing the exception. The variable "FileName" is passed as a parameter of type String to the web service. The list has attachments enabled. Can anyone spot what I am doing wrong?

            // open prototype site and web
            site = new SPSite(SPContext.Current.Web.Url);
            web = site.OpenWeb("cabinet");
            SPSite oSite = null;
            SPWeb oWeb = null;

            if (0 < Id.Length
             && 0 < FileName.Length)
            {
                try
                {
                    SPSecurity.RunWithElevatedPrivileges(delegate()
                    {
                        using (oSite = new SPSite(site.ID))
                        {
                            using (oWeb = oSite.OpenWeb(web.ID))
                            {
                                oWeb.AllowUnsafeUpdates = true;

                                SPQuery query = new SPQuery();
                                query.Query = String.Concat(
                                    "",
                                                "",
                                                "",
                                                "" + Id + "",
                                                "",
                                    "");
                                query.ViewFields = "";
                                query.RowLimit = 1;
                                SPListItemCollection listItems = oWeb.Lists\["ListName"\].GetItems(query);
                                // perform if item returned
                                foreach (SPListItem Item in listItems)
                                {
                                    try
                                    {
                                    String FilePath = String.Format("{0}{1}", Server.MapPath("~/\_doc\_uploads/"), FileName);
                                        FileStream fs = new FileStream(Fil