The file module allows extending the file field by calling the callbacks listed in $element['#file_value_callbacks'] within file_managed_value().
However, the $form_state variable is missing from the call, so the callbacks are unable to find the field or widget settings.
The attached patch fixes it.
Closed: won't fix
API documentation files
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
This is for the Drupal 7 Form API reference. Since Drupal 7 is past end-of-life this is now a "won't fix".
Work on documentation for form elements for supported versions of Drupal is at
📌
[meta] Move/Create Form Element Documentation
Active
and
🌱
[Meta] Improve documentation for Render and Form Elements
Active
.