ASP.Net Uploadify jQuery Plugin error message: cannot access closed file

I copied the code from here:

http://www.aspsnippets.com/Articles/How-to-send-email-with-Multiple-Attachments-in-ASPNet-Website.aspx

It worked when attach small files ( under 60 kb), but I got error (error message: cannot access closed file)when attach larger files.

Can anyone help? Many many thanks!

Hong