Source: dist/css/virtual_boot.css, line 8497
13.21.1 Attachments Big Button
On player pages use the big button for attachments. The text only displays when the view port is medium and up.
Example
Default styling
<a title="download attachments" class="btn btn-secondary btn-lg p-x_4 p_3 c_white" href="https://www.twitter.com/Tweat_DrSoda" target="_blank">
<i class="far fa-paperclip"></i>
<span class="display_none inline-block:md"> Attachments</span>
</a>