You mention that you are trying to enable the multimedia component in Render XSLT, and in the tags I see WebForms, so I assume its Render XSLT template component SDL Tridion WebForms.
WebForm is a presentation with a dynamic component, and the component template uses two XSLTs, one on the Expand XSLT tab, which runs during publication when the publisher displays the template code on the CMS. The dynamic template is different; it contains XSLT, which is copied at the time of publication as in Content Broker. When you request your view of WebForms components, the Dynamic Template is used to convert Expand XML to your WebForm.
Since the dynamic template is executed at the time of the request, at this point TcmScriptAssistant is no longer available (it exists only on the Content Manager side at the time of publication). The solution is to enable the Multimedia component in the Expand Template, using tcmse:PublishBinary() , in the Render Template, you can only convert / display the results of the Expand Template, but you do not have access to the CM functions at this point.
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