Thanks a lot! 
I can indeed now use:
---
title: {{title}}
authors: {{authorString}}
year: {{year}}
file: {{entry.files.[0]}}
---
And the file entry is correctly populated.
I am a bit puzzled as to whether I should put the file in the front matter or use instead
---
title: {{title}}
authors: {{authorString}}
year: {{year}}
---
file: [[{{entry.files.[0]}}]]
I would consider the file entry as metadata, but putting the file in the front matter does not make it clickable or findable by Obsidian (moreover, such file can be in an arbitrary sub-folder).
What is best practice here? A bit OT here I guess, probably I don’t fully grasp the meaning of the front matter…