[Hotfix] FilePostRedirectGet plugin and form collections

This PR fixes a problem with the `fileprg` plugin when using form collections with either `allow_add` or `allow_remove` set to true.

The fileprg invoked `Form::getInputFilter()` before `Form::setData`, so any filter provided by collection elements did not run at all for dinamically added/removed ones, since the number of elements/filters may vary on the basis of the input data.