The default content exporter should not put file attachments alongside the file entities themselves

Problem/Motivation

This fell out of #3633775: DefaultContent Finder fails with ImportException on non-entity .json files in content directory. When I sent that issue to @alexpott in Slack, he said:

Obvs it would be better if the exported files didn’t go in the same directory…

No lie detected. I think that'd be a good idea, if all file attachments all went to a specific place in a content export.

Proposed resolution

Figure out where attached files should go, and make sure the importer doesn't scan for content there.

Remaining tasks

TBD

User interface changes

None.

Introduced terminology

TBD, but none expected.

API changes

TBD, but I expect there'll be something here.

Data model changes

TBD

Release notes snippet

TBD