How to get delta of a file from within hook_tokens
It's the relationship between a file entity and another entity via a reference field that has a delta, not the file entity itself. A single file could, theoretically, be referenced by many entities, via multiple separate fields, or even twice in the same entity - meaning it doesn't conceptually have a single delta related to it.

What you'll need is to run an EntityQuery with sufficient contextual conditions to narrow down to one result, load up that parent entity, and loop through the field values until you find your file, giving you the delta. 

If your use case enforces that a file can only be referenced by a single entity, it'll be a lot easier to implement. But it would probably be safer to include at least some kind of limiting.