I want to upload a file from asp.net webpage to TFS server. Using TFS API I have created bug from my web page - but the bug also needs a file attachement to upload please let me
You should be able to simply create an Attachment object and add it to the Attachments collection on the WorkItem before you save it:
Dim attachment As New Microsoft.TeamFoundation.WorkItemTracking.Client.Attachment("c:\somefile.txt", "My Comment")
workItem.Attachments.Add(attachment)
workItem.Save()