How to use long file description?
The Inline Entity Form pushed me in the right direction.

Here's what worked for me:


Install File Entity module. It will allow you to add fields to files, in my case a "Description" textarea field.
Install File Entity Inline module. It will allow you to optionally include some of the fields directly in the file upload widget.
Add a file field and on configuration page under "Inline fields" choose either "All available fields" or "Only selected fields".


That's it - hope this helps someone!