The files->source field is supposed to ALWAYS contain some string that indicates where the file came from. For many repositories (like URL or Wikimedia) it should have the full URL to the original file. For private repositories (like Equella) it can just have the original filename like "somefilename.jpg"

These can be used to produce the human-readable source in the fileinfo dialogue in the file manager, both for referenced files and potentially also for files that have been copied into Moodle.

I've been exploring this in 2.3 and finding that:

1) It's really hard to see how this field should be set. Please explain exactly where and how.
2) When it is set, we sometimes see a bare URL and sometimes see a serialised array.
3) It's not set at all for many repositories (like Equella).

I suggest that we stick to ALWAYS using a URL to the source (either the source file itself or to a page where that source file can be found).

Someone needs to go though and fix up all the repositories to behave consistently.