Using multiple controls in a modal popup ASP.NET

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ASP.NET

ajaxToolkit:ModalPopupExtender

selectedIndexChanged

My task is to use a Modal popup Extender to change the text value of text box from the selected text of a Listbox. This has been accomplished, as I use a button to take the selected text and place it in a text box. The text I show in the Listbox is a list of file names of images used in the application, so my next task is to to use the Filename text in the Listbox to show the image that relates to the file listed. I am wondering how to do this either server side or client side. The modal and panels code is all inside an updatepanel.

What I have tried:

I have tried using the selected index change and server side but the event does not register! I am resuming that is because the modal extender fires from the button I use to show the modal via the TargetControlID. I have yet to try any javascript and the BehaviourID. I have yet to try a JS as my list is populated server side.