Sharepoint is adding the attachment itemattachmentadding event.

Itemattachmentadding is the pre-event processor that is adding an attachment to the item, that is, the event triggered before the attachment is added to the list. You can add operations in the itemattachmentadding event before the attachment is added to the list.

Adding an attachment to the SharePoint 2010 list is added when the list item data is added. You can directly add an attachment to each list item in the "New Project" window, as shown in 1:

Figure 1

In the ribbon of the new project window in Figure 1, there is a feature of "additional files", click it, as shown in 2: See full text: http://www.360sps.com/item/ba9d21984853419e8bf0084b76ee5937.aspx