Create a custom Link to a File-Field in Twig
After more research, i found the answer by accident on this page. One can refer to the link by using: 

&lt;a href="{{ file_url (node.field_publication_file.entity.uri.value) }}"&gt; 
  Download
&lt;/a&gt;


The fields that were defined in the bootstrap theme don't seem to be accessible in a sub-theme of bootstrap. I'm sure I could have written a custom preprocessor to do the same thing, but this solution works for me and I don't have to write PHP code.