The $file_update
array is associative, therefore it doesn't have integer keys, its keys are strings.
$file_update[0]
does not exist and this statement should throw an error given you have proper error reporting configured.
If you want to access specific array values use:
$file_update['file_id'];
$file_update['file_update'];
If you want to access the first element of an associative array you can use the following:
$key = array_keys($file_update)[0];
$value = $file_update[$key];
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