receipts/view: Display PDFs in iframe

PDF attachments can now be shown on the receipt view page in an
`iframe`.

templates/receipt.html.tera

@@ -11,9 +11,18 @@
       margin-right: 1em;
     }
 
+    .photo {
+      flex-grow: 1;
+    }
+
     .photo img {
       max-width: 100%;
     }
+
+    .photo iframe {
+      width: 100%;
+      aspect-ratio: 8.5 / 11;
+    }
   }
 </style>
 {% endblock %} {% block main %}
@@ -52,7 +61,11 @@
   </div>
   <div class="photo">
     <p>
+{% if filename is ending_with(".pdf") %}
+      <iframe src="/receipts/{{ id }}/view/{{ filename }}"></iframe>
+{% else %}
       <img src="/receipts/{{ id }}/view/{{ filename }}" />
+{% endif %}
     </p>
   </div>
 </article>