Simplify showing/hiding attached file name

Note we have to render the `<p>` tag in one line because at the time of
writing browsers don't consider elements with whitespace inside as empty
[1].

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/:empty

app/components/documents/fields_component.html.erb

@@ -17,9 +17,7 @@
   </div>
 
   <div class="small-6 column">
-    <p class="file-name">
-      <%= document_attachment_file_name %>
-    </p>
+    <p class="file-name"><%= document_attachment_file_name %></p>
   </div>
 
   <div class="small-12 column">