Allow different stores to be set post files versus media files

[paulrobertlloyd]

Posts filed are small text files; typically something that you would want to host using Git so that you can track changes.

Media files are large binary data; typically something you don’t want to host in a Git repository if you can help it. These are better hosted on dedicated object/bucket storage.

Currently you can only choose one type of content store; might it be possible to configure different stores for posts and files?

Likely a requirement for #14 and #532.