I recently encountered a problem with ASP.NET whilst using a file upload control. Initial thoughts were that it was related to the AJAX extensions I was using, however after a lot of playing about, i found adding the following line to the web.config file resolved the problem,
Basically the problem is caused by an ASP.NET limit of around 4MB for a file upload, the above setting tells ASP.NET to allow larger files to be attached / uploaded.
Hope this helps someone.