Its fine that you have found how to pass the parameters in the documentation. So the rest is easy, if the ComboBox (for instance x:named "combo") and the RadUpload are placed in one xaml page you can get the selected ComboBoxItem with
this.combo.Selecteditem as RadComboBoxItem
in the event handler from which you wish to pass parameters. Please feel free to ask if you find difficulties in implementing this.
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