I had a problem today using a multipart form which gave me the following error: “Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true... in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.Registe...