I just went through this, my working space is a mess! I ordered this, it should be here tomorrow. :)

https://www.amazon.com/gp/product/B00R6WWCFG/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 You can add another set of 3 drawers on top and have the file holder piece on its own.

I'd considered these, but will instead clear out an old one I have, no rollers on it though, which would be nice.

https://smile.amazon.com/IRIS-4-Drawer-Organizer-Top-Casters/dp/B00447NWFK/ref=sr_1_6?ie=UTF8&qid=1465773751&sr=8-6&keywords=storage+organizer+cart

https://smile.amazon.com/IRIS-6-Drawer-Storage-Rolling-Organizer/dp/B00ISOM8ZK/ref=pd_sim_201_1?ie=UTF8&dpID=31KGiD9EWzL&dpSrc=sims&preST=_AC_UL160_SR95%2C160_&refRID=HEFXVH8X3H1DEJY9FZ6Z