1) Find a tutorial on how to create objects with eZ API in general. That's a must.
2) Learn a bit about how datatypes are created.
3) Each content object attribute of different datatype may have its own way of storing data, and File datatype will most likely do. List your file class attributes, and for each one, go to /kernel/classes/datatyles/* and learn how it fetches/stores data, and use that API. 4) Don't forget to publish the newly created object/node.