:) OK hyperlink.
So a hyperlink basically consists of the URL to go to, which you have mastered, plus some words to click on. So we need the words. We can make use of the recent December 2018 feature to summarize text up to a parent.
Make a formula text field on the child record as ToText([my file attachment field])
You have the [Maximum Record ID#] field up on the Parent. Look that up down to the children.
Make a Summary Combined text field on the relationship to summarize the file name words, subject to the filter that the [Record ID#] = [Maximum Record ID#]
(ie there will only be 1 lucky qualifying record to Summarize)
Finally make the Hyperlink field as a formula Rich Text field.
var text URL = [insert the URL field that you previous got working];
var text Words = ToText([My Summary Combined text field]);
"<a href=" & $URL & ">" & $Words & "</a>
