Arguments of hook_file_download_access_alter()
You do not need to use $field, $entity_type and $entity in your implementation of hook_file_download_access_alter(), if you do not want to.

You should use theses arguments if you need to handle files differently depending on which field of which entity they belong to.

For example, node_file_download_access() restricts access on files to people that can view the node. If you want to remove this restriction for nodes of a specific type foo, you would have to check whether $entity_type == 'node'and whether $entity-&gt;type == 'foo' and grant access if this is the case.