Allow referencing an existing file for field mapping

Problem/Motivation

The PDFs we would like to parse are being attached to content already (via a media field in this case), and rather than having to download and re-upload we should be able to reference the already-existing file and do the field mapping.

Proposed resolution

Related to #3040914: [8.x-5.x] Support Media in Core → and perhaps #3038956: Allow reparsing a PDF file without reuploading it →

Remaining tasks

User interface changes

API changes

Data model changes