Either you are doing something wrong when you send the email (I don't know how you are currently sending it) or there is a problem with the way you have set up IFTTT.
According to our tests, it should be possible to configure IFTTT so that the text "message:" is not added to the file.
It is up to you to discover why the text is being added.
If you are not able to get rid of this text, then just use an event instead. Change the radio button in Watcher settings to generate an event and then assign that event to a command that does the TTS (and anything else you want to do when motion is detected).
Honestly, I don't know why you don't just use Watcher plugin to monitor a folder that your camera saves video files to. It would be much easier and would work even if your internet is not working.