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It would seem like the easiest thing to do is get
urllib.urlencode into what ever namespace "python:" draws from

Then it gets fuzzy because I'm don't grok TAL.  Somehow change:

    <a tal:attributes="href string:file${file/id}/${file/name}"
       tal:content="file/name">dld link</a>

To use python:urlencode(${file/name})

I'm remarking this bug as open since you seem to agree it's
a problem.