I'm getting this error also, how do you disable MimeTypeChecking in a MVC app? I need this to support .AI files.

This does not work:

public override UploaderValidateOption GetValidateOption()
{
        var option = new UploaderValidateOption
        {                            

          MaxSizeKB = Settings.Instance.MaxSizeKb,
          EnableMimetypeChecking = false
        };
        return option;
}