For file drawers, you need heavy-duty slides -- rated at least 100 lbs. whether side-hung or under-mounted. You also want full-extension (or even full-extension with 1" overtravel) slides, so you have full access to the contents at the back of the drawer when fully extended.
When I was a product manager at an office furniture manufacturer, we used Accuride slides -- the double-height units with overtravel and steel ball bearings (not the Delrin bearing models). I've also used Grant slides, which are virtually exact equivalents of the Accurides (or used to be -- I haven't checked recently).
Lately, for kitchen cabinets, I've turned to the Blum tandem Blumotion undermount slides, mostly for appearance reasons, but also for the soft-close feature. They're more expensive than typical side-mounted slides and IMO overkill for file cabinets.