app/views/hyrax/file_sets/_thumbnail.html.erb
<div class="thumbnail file-set">
<img src="<%= presenter.solr_document.thumbnail_path %>" />
<div class="caption">
<%= link_to hyrax.download_path(presenter), class: 'btn btn-success btn-block' do %>
<span class="fa fa-download"></span>
Download
<% end %>
</div>
</div>