The file is uploaded, but it is not attached to the issue.

Using the file widget on the issue form does work. This is
using Mozilla.

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user_id=4923

Found it.
cgi_client.Client._post_edit_node, in the ":multilink" branch,
the line:
  value = link.get(nodeid, property)
should be
  value = link.get(nodeid, property)[:]

Otherwise node.set will see it "already" set.

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user_id=555791

Ok, Goron's patch works if I use the standard CGI interface.

It still fails through Zope.  I did some playing around with
it -- in 2 locations, the form has the empty string as the
key when it should be :multilink.  The current code bombs
(IndexError) when the empty string is found.  Any ideas on
why the form gets hosed by zope/zroundup?

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Patch applied to CVS, thanks.   
   
Sorry, but I don't think that'll ever work in Zope. You can 
make it work by forcing the form to be a GET form, but by the 
time the form contents get to ZRoundup in a POST, the ':foo'   
parameters are swallowed whole by Zope's ':type' argument   
magic handling.   
 

Logged In: YES 
user_id=4923

Found it.
cgi_client.Client._post_edit_node, in the ":multilink" branch,
the line:
  value = link.get(nodeid, property)
should be
  value = link.get(nodeid, property)[:]

Otherwise node.set will see it "already" set.

Logged In: YES 
user_id=6405

I've applied that patch already.