Refactor requestDataProcessor for File out into its own file

[rattrayalex-stripe]

r? @richardm-stripe
cc @ob-stripe @stripe/api-libraries

This was in a codegen'd file, which lead to big/ugly diffs; we should ~never have substantial application logic in a codegen'd file.

While I was looking at the code, I refactored to make it a bit more simple/modern/unified.

[richardm-stripe]

lgtm
Seems much better to me and, as far as I can reason, does the same thing. I'd be even more happy if there were a test.

[rattrayalex-stripe]

FWIW there are some high-level integration tests in test/resources/Files.spec.js but that's certainly fair, coverage here is probably not what it should be.