How to use file upload type field within subfields type in Laravel backpack 5

Question

I have a relation as follows: A Class has many ClassContents.

The code at Class model as follows

public function classContents(): HasMany
{
    return $this->hasMany(ClassContent::class);
}

The code at ClassCrudController's setupCreateOperation as follows

$this->crud->addField([
    'name'          => 'classContents',
    'type'          => "relationship",
    'subfields'   => [
    [
         'name'      => 'attachment',
         'label'     => 'Attachment',
         'type'      => 'upload',
         'upload'    => true
    ],
... ... ... 

The code at ClassContent model as follows

public function setAttachmentAttribute($value)
{
    $attribute_name = "classContents['attachment']";
    $disk = "public";
    $destination_path = "class_contents";

    $this->uploadFileToDisk($value, $attribute_name, $disk, $destination_path, $fileName = null);
}

File upload does not work with the above arrangement. Please suggest.