Andrew,

 

This error means the ASP.NET can't parse the http posted data to Forms/Files collection.

 

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Please try UploadType=Flash and UploadType=Silverlight

 

Now I guess that is a bug when use Silverlight to upload files.

 

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Is your website using Medium trust level ? or are you using CuteWebUI.AjaxUploader.UseAspNetStream ?

 

Regards,

Terry