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.
