/// <summary>

/// Asynchronous event occurs after the attachment is added

/// </summary>

/// <param name="properties"></param>

public override void ItemAttachmentAdded(SPItemEventProperties properties)

{

    base.ItemAttachmentAdded(properties);

    // Make sure we have at least one attachment (this should never happen, but just in case)

    if (properties.ListItem.Attachments.Count == 0)

    {

        return;

    }

    // Get a reference to the attachment

    SPFile attachment = properties.ListItem.ParentList.RootFolder.SubFolders["Attachments"].SubFolders[properties.ListItemId.ToString()].Files[properties.ListItem.Attachments.Count - 1];

    // Get the raw data of the file

    byte[] content = attachment.OpenBinary();

    // Get the original attachment name

    string originalFileName = attachment.Name;

    // Get the new file name with the date/time stamp

    string newFileName = GetFileNameWithDate(originalFileName);

    // Turn off events so that we don't get an infinite loop when we add back the attachment

    DisableEventFiring();

    // Delete the original attachment (we can't simply rename the attachment)

    properties.ListItem.Attachments.DeleteNow(originalFileName);

    // Add the attachment back with the new filename

    properties.ListItem.Attachments.AddNow(newFileName, content);

    // Turn on events again

    EnableEventFiring();

}

/// <summary>

/// Prefix a file name with the current date time

/// </summary>

/// <param name="fileName">The original file name</param>

/// <returns>A file name prefixed with the date time (e.g. 2008010101010101_test.docx)</returns>

private string GetFileNameWithDate(string fileName)

{

    DateTime now = DateTime.Now;

    return 

        now.Year.ToString() + 

        now.Month.ToString().PadLeft(2, '0') + 

        now.Day.ToString().PadLeft(2, '0') + 

        now.Hour.ToString().PadLeft(2, '0') + 

        now.Minute.ToString().PadLeft(2, '0') + 

        now.Second.ToString().PadLeft(2, '0') + 

        now.Millisecond.ToString().PadLeft(2, '0') + 

        "_" + fileName;

}

    }