SharePoint list attachments, how to overwrite them

Quick question about SharePoint.... I need to update an attachment on a list using the SharePoint sdk,but when ever I delete the old one, and add the new one, the new document is never added. Below is my code...

/* Delete the attachment first and create a new attachment.*/
            string fileName = newAttachmentName.Substring(0, newAttachmentName.IndexOf("."));
            //Delete Attachment
        开发者_StackOverflow社区    SPAttachmentCollection attachments = item.Attachments;
            if (item.Attachments != null)
            {
                string oldfilename = attachments[0].ToString();

                attachments.DeleteNow(oldfilename);
                item.Update();
            }

            //AddAttachement(item, newAttachmentName, attachmentStream, true);
            attachments.Add(newAttachmentName, contents);
            ////attachments[0] = filename;
            item.Update();

Maybe this will be of any help:

Retrieving Attachments from SharePoint List Items

Below does NOT work:

SPAttachmentCollection attachments = listitem.Attachments;

foreach (SPFile file in attachments)
{
  // Do something
}

Below DOES work:

SPFolder folder = web.Folders["Lists"].SubFolders[list.Title].SubFolders["Attachments"].SubFolders[listitem.ID.ToString()];
foreach (SPFile file in folder.Files) 
{   
    // Something useful here 
}

SPAttachmentCollection attachments1 = item.Attachments;
attachments1.Add(newAttachmentName, contents);
item.Update();``

where you overwriting your new file create new instance of Attachments( attachment1)