add unittest for external images

src/paperless_mail/tests/samples/sample.html

@@ -4,7 +4,11 @@
     </head>
     <body>
       <p>Some Text</p>
-      <p><img src="cid:part1.pNdUSz0s.D3NqVtPg@example.de" alt=""></p>
+      <p>
+				<img src="cid:part1.pNdUSz0s.D3NqVtPg@example.de" alt="Has to be rewritten to work..">
+				<img src="https://upload.wikimedia.org/wikipedia/en/f/f7/RickRoll.png" alt="This image should not be shown.">
+			</p>
+
       <p>and an embedded image.<br>
       </p>
       <p id="changeme">Paragraph unchanged.</p>