Fixing filenames that can not be converted to ByteString

When I tried to use jack_up to upload files with names, that are not english I got an error in FF
``` 
Cannot convert string to ByteString because the character at index 0 has value 1087 which is greater than 255
```
And in chrome I got
```
Uncaught SyntaxError: Failed to execute 'setRequestHeader' on 'XMLHttpRequest': 'привет!.jpg' is not a valid HTTP header field value. 
```

This is a small fix, that works for me.