Sorry Chris, is this on the phone or the Linux box (as I'm not in front of either to be able to check).
FWIW I just plugged my SGS4 into my Ubuntu 14.04 and it first came up with this (which is similar to what I saw on my mates Mint setup remotely last night):
... but then a file browser window opened showing me the phone and uSD card (that I could explore fine)?
(Copying that desktop snapshot to the Dropbox folder instantly reminded me of one of the trivial but_oh_so_annoying issues I have with Linux and that it whilst in detailed view I couldn't paste the file into the folder. I had to change to icon view to do that.
Cheers, T i m