server side error when file field inside group

there is a bug when a file field is inside a hidden group.

This will not work (throws 500 ERROR server side):
```
[text a]
[group g]
  [file file-2]
[/group]
[submit]
```

Temporary workaround for now:

Change file fields inside groups to required file fields, like this:

```
[text a]
[group g]
  [file* file-2]
[/group]
[submit]
```

Then it will work.