List Attachments over 50MB need more than an increase in Maximum Upload Size... - Brett's SharePoint Blog - Site Home - MSDN Blogs

http://blogs.msdn.com/b/bgeoffro/archive/2008/03/19/list-attachments-over-50mb-need-more-than-an-increase-in-maximum-upload-size.aspx

Basically there's a setting maxRequestLength in web.config file at the IIS level that will block list attachment uploads above 50MB. maxRequestLength="204800" would allow list attachment up to 200 MB.