fix for current preroll campaign

Some campaigns have mp4 files marked as video/x-mp4 which breaks the player. Have tested and it seems to be compatible as video/mp4 so mapping to that.
guessing we might have more than one 'bad' mime type mapping to a 'good' one, i.e. would it be better in the format

```
formatMap: {
    'video/mp4': ['video/x-mp4', 'x-video/mp4']
}
```

Does mean you'll need to lookup in the object's value rather than key, but should be lodash functions for that