Having to send a file browser to /usr/bin to find an executable to handle a file type is insane. There are 3240 files in that directory on my system. How a non-expert user is supposed to cope is beyond me--if they don't give up after Firefox 'crashes' (freezes while stat'ing all the files in there) first.
Compare with the user experience in Epiphany, which actually makes an effort to integrate with the GNOME desktop; the user is presented with the option to open the file with their default handler, or save it to disk. The default handler is determined via the freedesktop.org MIME spec, which is the same thing used by everything else in GNOME. An administrator may change that for a particular user, or all users, using the standard Unixoid methods outlined in the spec.
Where Epiphany falls down is in letting the user change their preferred handler for a file type; there is no UI for that yet. So the user would have to save the file, then change the handler for it in Nautilus. Eventually, hopefully Epiphany will be enhanced to ask the MIME database who could handle the MIME type it's been served, and give the user a nice list of all the programs that can handle it, in the same way that Nautilus does. Nothing's perfect, eh? :)