maxJsonLength error - string too long

The following error pops up when adding an attachment > +/- 1.5 MB using the file open dialog from the sample below.

http://support.geocortex.com/attachment-manager-workflow-with-file-open-dialog

"Error during serialization or deserialization using the JSON JavaScriptSerializer. The length of the string exceeds the value set on the maxJsonLength property"

Is it possible to increase the value of maxJsonLength somewhere ? In a web.config?