Support updates with multiple image files

As of version 2.9.0 if you run

```
t update 'This is a test' -f /tmp/1.png -f /tmp/2.png`
```

`t` reports success but only /tmp/2.png is posted. I expected the command to either attach both files to the tweet or to return an error message that said you could only post one file at a time.
