[Media] EncoderPlugins should handle the views that concern them

Ensure the intended filetypes and mimetypes during Vips conversions (part 2)
Sanitize Attachments instead of Validate (part 2)
Various bug fixes

plugins/ImageEncoder/templates/imageEncoder/imageEncoderView.html.twig

@@ -0,0 +1,8 @@
+<div>
+    <figure>
+        <img src="{{ path('attachment_thumbnail', thumbnail_parameters) }}" alt="{{ attachment.getTitle() }}">
+        <figcaption><a
+                href="{{ path('attachment_show', {'id': attachment.getId()}) }}">{{ attachment.getTitle() }}</a>
+        </figcaption>
+    </figure>
+</div>