I am trying to add an attachment to new item but I could only succeded to upload the attachment into the list's folder (not to list's attachment folder).
How can I achieve uploading the attachment on creating new item process?
I had the same problem before. The thing is that sharepoint creates the folder when you add your first attachment, not on creating the item. In other words, the folder you are trying to upload to does not exist. You can create this folder by either adding a dummy-file and removing it in an ItemAdding EventReceiver when you create the item. Another alternative is to create the folder from SL COM. Just make sure that it has the item-id as folder-name and put it under attachments. Hope this will help you :)