Naming conventions in Media entity vs File entity
After a bit of reading around, it seems that the core Media module was never adapted/updated from the File Entity module; it was a port of the Media Entity module (which didn't have its origins in File Entity either). See Bring Media entity module to core as Media module for the full details.

The "file" media type did have its origins in a plugin named "Document", provided by the Media entity document module, which provided the same basic functionality as File Entity's "document". So there may have been some crossover in their evolutions somewhere along the line.

But even so, reading through the proposal to Add "File" MediaSource plugin, it becomes clear that there was no intention to port the "document" type over in its previous form, rather just to use it as a starting point for the generic file type. From comment 24 on that issue, in response to a previous query of I'm wondering whether file should be document instead? File seems really generic as image is also a file:


  26-1: As commented with @dawehner on the sprint, the idea is that the Image handler extends this plugin, so this name is probably the most appropriate in this case. A "Document" plugin can be created also extending "File", if needed to expose specific UI improvements for documents.