[Blueprint] Can't detect file destination if the file field wrapped with fieldset under list field.

--- YAML File

```yaml
'@extends': default
form:
fields:
tabs:
fields:
content:
fields:
header.media_order:
unset@: true
header.test1:
type: list
fields:
.photo:
type: fieldset
title: Photo
fields:
.imagetitle:
type: text
label: Image Title
.imagefield:
label: Photo
type: file
style: vertical
destination: 'self@'
limit: 1
multiple: false
accept:
- image/*
.video:
type: fieldset
title: Video
fields:
.videotitle:
type: text
label: Video Title
.videourl:
type: text
label: Video URL
```