How to match a WiredTiger index file to its corresponding collection
Ref:
https://docs.mongodb.com/manual/reference/method/db.collection.stats/

https://docs.mongodb.com/manual/reference/command/collStats/#collStats.indexDetails

As you are using WiredTiger storage engine you can use db.collection.stats() with indexDetails option set to true.

Easy copy paste:

.stats({"indexDetails":true})


  A document that reports data from the WiredTiger storage engine for
  each index in the collection. Other storage engines will return an
  empty document.
  
  The fields in this document are the names of the indexes, while the
  values themselves are documents that contain statistics for the index
  provided by the storage engine. These statistics are for internal
  diagnostic use.