The URL should be:

https://westeurope.azuredatabricks.net/files/my-stuff/my-file.txt?o=1234567890123456

Note that the file must be in the filestore folder.

As a side note I've been working on something called DBFS explorer to help with things like this if you would like to give it a try? https://datathirst.net/projects/dbfs-explorer/