[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.

`FilePostRedirectGet` 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.

Edit:
I also added some more code to fix another problem that occurs in the plugin, indicated in #5381.

When a collection target element is a fieldset with both a file and normal input, the merge done by `array_merge_recursive` (used by FilePRG) leads to unpredictable behaviour due to colliding numeric keys in both `$_POST` and `$_FILES`.

#5244 (which in turn fixes #4974) adds the capability to preserve numeric keys in `ArrayUtils::merge()`

#5381 was supposed to test the new behaviour but it was somehow borked, so i started from @2DivisionsByZero work to provide a comprehensive PR.