// Gets the first page located under /Articles
TreeNode page = new DocumentQuery<TreeNode>()
.Path("/Articles", PathTypeEnum.Children)
.OnSite("MySite")
.Culture("en-us")
.TopN(1)
.FirstOrDefault();
if (page != null)
{
DocumentAttachment attachment = null;
// Prepares the path of the file
string file = System.Web.HttpContext.Current.Server.MapPath("/FileFolder/file.png");
// Inserts the attachment into the 'ArticleTeaserImage' field and updates the page
attachment = DocumentHelper.AddAttachment(page, "ArticleTeaserImage", file);
page.Update();
}