This issue is very similar to
[PSR-7] Files behavior wrong description (which has been fixed in the spec as far as I can tell):
The $_FILES code sample for
<input type="file" name="my-form[details][avatars][]" />
is inaccurate. It states that the according $_FILES structure looks like this:
array (
'my-form' => array (
'name' => array (
'details' => array (
'avatar' => array (
0 => 'my-avatar.png',
1 => 'my-avatar2.png',
2 => 'my-avatar3.png',
// ...
But according to the input name, the nested key should be avatars instead of avatar.
Or am I missing something?