mindmap macro fails to generate a URL with the server port number

I run a XWiki installation on a non-standard port (in my case 8080).
I put an freemind *.mm file as an attachment to a xwiki page and put
the #mindmap macro on the page (e.g. #mindmap("example.mm", "100%", "500").

After saving the page, the freemind applet alert an error message with
a wrong URL to access the attachment. The port number is missing.