Allow devs to enable CP for attachments

[justintadlock]

I disabled content permissions for attachments because of confusion between "post" and "file" from users. However, developers should be able to re-enable this.

The existing filter hook accounts for this in admin/class-meta-box-content-permissions.php:

return apply_filters( "members_enable_{$post_type}_content_permissions", true );

However, a later check overrules this in the add_meta_boxes() method:

if ( 'attachment' !== $type->name && $type->public )
	add_meta_box( 'members-cp', esc_html__( 'Content Permissions', 'members' ), array( $this, 'meta_box' ), $post_type, 'advanced', 'high' );

Related topic: https://wordpress.org/support/topic/member-roles-on-media-attachments/