add attachments notification (#6769)

webapp/i18n/en.json

@@ -1127,6 +1127,7 @@
   "channel_loader.unknown_error": "We received an unexpected status code from the server.",
   "channel_loader.uploadedFile": " uploaded a file",
   "channel_loader.uploadedImage": " uploaded an image",
+  "channel_loader.postedImage": " posted an image",
   "channel_loader.wrote": " wrote: ",
   "channel_members_dropdown.channel_admin": "Channel Admin",
   "channel_members_dropdown.channel_member": "Channel Member",