thread 'main' panicked at 'open input file path: ()'

Sometimes I get the following error message when batch compressing files:

```
thread 'main' panicked at 'open input file path: ()', ~/.cargo/registry/src/github.com-1ecc6299db9ec823/imager-0.3.2/src/main.rs:150:73
```

I assume it's due to a wrong file type or something.