Use System.Web.UI.WebControls.FileUpload control is named as fileUpload in code.

Sample code for uploading the file from Local Machine to ListItem attachments as follows,

//Check the FileUpload control has the file

if (fileUpload.PostedFile != null)

{item = list.Items.Add();

item["Title"] = fileUpload.FileName;

//Read the Contents from the file in Local machine

Stream fs = fileUpload.PostedFile.InputStream;                   

byte[] fileContents = new byte[fs.Length];

fs.Read(fileContents, 0, (int)fs.Length);

fs.Close();

// Add the file to the ListItem as an Attachment

SPAttachmentCollection attachments = item.Attachments;

string fileName = Path.GetFileName(fileUpload.PostedFile.FileName);

attachments.Add(fileName, fileContents); 

item.Update();                                           

}