[Zope] Using DTML in filename_expr=" "

This should be simple, but has escaped me. What is the syntax to insert a variable into ""? This is to name the file attachment to an email. I have tried the entity syntax(reserved for HTML), but nothing works. <dtml-boundary type="application/octet-stream" disposition="attachment" encode="base64" filename_expr="'&dtml.-filename;'+'.vcf'"> TIA, -Trevor