i18n 'Media message'

Fixes #736

// FREEBIE

_locales/en/messages.json

@@ -269,5 +269,9 @@
     "upgrade": {
         "message": "Upgrade",
         "description": "Label text for button to upgrade the app to the latest version"
+    },
+    "mediaMessage": {
+        "message": "Media message",
+        "description": "Description of a message that has an attachment and no text, displayed in the conversation list as a preview."
     }
 }

js/models/messages.js

@@ -73,7 +73,7 @@
                 return description;
             }
             if (this.get('attachments').length > 0) {
-                return 'Media message';
+                return i18n('mediaMessage');
             }
 
             return '';