Populating file fields with an http-request with the Services module and a REST Server (Drupal 7)
Currently services doesn't offer the ability to add a file to a node in ONE method call, but thats easily within the scope of what services can offer, its just not in services core.

Currently you can make a request to create a file and get the returned FID, then you can process to submit a node with the field structure for a file field.

Which is something like

{"nid":1, "field_image":{"und":[{"fid":2}]}}

Forgive me thats very rough, but you can can use devel to look at an already existing node with an image attached and mimic that in your submission and it will work.

The current process is:

POST to /file

get returned FID

PUT to /node make sure to have nid and field_* in the correct structure or it won't stick.