Add hint to file previews not supported by the viewer

Like done on links, " ↗" is now added to the name of file previews to
hint that the file will be opened in a different tab/window.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

src/components/MessagesList/MessagesGroup/Message/MessagePart/FilePreview.vue

@@ -22,6 +22,7 @@
 <template>
 	<a :href="link"
 		class="container"
+		:class="{ 'is-viewer-available': isViewerAvailable }"
 		target="_blank"
 		rel="noopener noreferrer"
 		@click="showPreview">
@@ -183,6 +184,12 @@ export default {
 		force it to be on its own line below the preview. */
 		display: block;
 	}
+
+	&:not(.is-viewer-available) {
+		strong:after {
+			content: " ↗";
+		}
+	}
 }
 
 </style>