refs https://github.com/TryGhost/Toolbox/issues/523
- To support generic "file" import handling the content file handler needs more configuration options for properties like type, extensions, content path etc. This refactor makes the handler configurable and reusable for any type of file import
refs https://github.com/TryGhost/Toolbox/issues/523
- We need to process generic files like .pdf, .md, etc. on the importer "handler" stage.
- The media handler can process more than just media files after few refactorings. Renaming it to a generic content file handler indicates it can process any type of content file.
- In future we can substitute the built-in "images" import handler with this generic content file handler.