we are using the default qt file dialog and on my machine alt+up goes to the parent folder.

That's definitely not happening here.

You made me think, though -- I use an xmodmap script to put ctrl as close as possible to the space bar. Maybe that was interfering? It turns out, the answer is no. I unplugged my USB keyboard and plug it back in again (which resets the modifier mappings to the default), no difference. On my machine (for whatever reason), alt is completely ignored in the file dialog -- that is, if I focus the file list, alt-up and alt-down just move up and down in the file list.

I'd be happy to help debug.

on my machine (kde) this seems to be configurable in the 'standard keyboard shortcuts' of the system settings. 'up' is mapped to Alt+Up

closing. for now, we keep using the qt's file dialog.