support selective file exports with select[files] param

this is a friendly alias for select[attachments], done to be
more consistent with existing API names, and following the
example of select[modules], select[pages], etc.

test plan:
 - do selective content exports with the content export API,
   using select[files][]=(id), and ensure the correct file(s)
   are exported
 - test both common cartridge and zip exports

fixes CNVS-15129

Change-Id: I004ae1e8ba28ea8b7cf480c8fee5bd9633375781
Reviewed-on: https://gerrit.instructure.com/40115
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>