Slow day at work, and your question got me curious again myself. I tried a while ago and could never work it out.
I think the xdg-utils extension is broken.
1. If you're not using gnome or KDE, xdg-mime EXPECTS to find the `file`command in /usr/bin - Which means file.tcz should probably be a dep, and it's not in /usr/bin anyway.
2. xdg-open is a bit limited. if you aren't running gnome/kde/xfce, it tries to open your file in the first browser it finds (of the selection firefox, mozilla, netscape), rather than whatever you've told xdg-mime you want to use.
So a "Wow, this really doesn't work the way I want it to" solution if you aren't using XFCE, Gnome, or KDE goes something like this (You probably just want to skip down to the alternative below):
1. install xdg-utils.tcz
2. install file.tcz
3. install firefox.tcz
4. cd /usr/bin; sudo ln -s /usr/local/bin/file
5. Download something in chrome, click on it, and it will open in firefox! (hahaha fail)
Alternatively, untested:
doesn't work the way I want it to" solution goes something like this:
1. install xdg-utils.tcz
2. install file.tcz
3. install xfce.tcz
4. cd /usr/bin; sudo ln -s /usr/local/bin/file
5. xdg-mime default <application> <mimetype> (ie xdg-mime default evince application/pdf) (xfce might even provide it's own defaults if you're lucky)
6. Download a PDF in chrome, and click on it.