filepicker in elements IN a list does not work

While nesting a filepicker (or pagemediaselect) in elements OR list is possible, nesting them in elements IN a list is not.
A warning message «Not found» appears in the top right corner. (Tested in Grav v1.7.33 w/ Admin v1.10.33.1)

```
form:
fields:
tabs:
fields:
content:
fields:
content:
unset@: true
header.list:
type: list
label: List
style: vertical
fields:
.name:
type: text
label: Name
.pagemedia:
type: pagemediaselect
label: Pagemediaselect in List
preview_images: true
.file:
type: filepicker
folder: 'page://images/'
label: Filepicker in List
preview_images: true
accept:
- .jpg
- .png
.type:
type: elements
label: 'Type'
size: small
default: text_only
options:
text_only: Text only
text_and_image: Text and image
fields:
text_only:
type: element
fields:
.text:
type: text
label: Text (One)
text_and_image:
type: element
fields:
.text:
type: text
label: Text (Two)
.file:
type: filepicker
folder: 'page://images/'
label: Filepicker in List in Elements
preview_images: true
accept:
- .jpg
- .png
.pagemedia:
type: pagemediaselect
label: Pagemediaselect in List in Elements
preview_images: true
```