MQTT: provide filename of recording

[gdoumen]

MQTT support is a great improvement.

However, whenever I receive that message ( e.g. motion_stop), I would like to also retreive the recording and potentially present it to the user ( e.g. downloading the file via FTP, store it in my own cloud/server, send a push notification to user with link to the video)

However with the current messages/payload, I don't get this information and I would have to guess the filename (based on timestamp)