How to implement access permissions per individual field and individual nodes?
Not sure if it is going to be the best/most elegant solution, but here is "an" approach that might work:


Use the Entity reference module to replace your file field in your existing content type with a reference to some other content type (say Resource).
Attach your files to nodes of content type Resource (so not in your already existing content type nodes).
Use the Content Access module to implement the refined access that you are looking for, but applied to that Resource content type.
Create some testcase(s) to verify how that refenced Resource node is going to be shown in your original nodes.


Get the idea? Pretty basic Drupal techniques (to KeepItSuperSimple, aka KISS).

I haven't tested it (yet) while writing this answer, but I bet functionally it will behave exactly as what you'd want. And guess what: no need for the field permissions module to make this work.

FYI: the above is a D7-equivalent of how I used to create such kinds of related content in D6 already (also using the Content Access module, and I believe in D6 it was "node reference" or something like that).