It's quite simple to pick open a file cabinet. Most file cabinets nowadays have a cheap disk tumbling system. If the disk patterns match the key shape, presto! It's open.
The technique shown here for opening locked door is the most basic. For this it has to be a door with a gap as wide as a width of a butter knife.
You are going to learn most the simple ways to pick file cabinet locks and open locked doors by reading this file.