How to set a upload directory for file_upload

Thanks Anh, that example works. A bit too limited for our needs though.
It gets a lot more complicated since we're trying to do that only for a specific custom post type, not complete WordPress.

We basically created a CPT "Downloads" and each post has a file uploaded to it through the file_upload metabox field. This file needs to be uploaded to /wp-content/uploads/secure/ and not /wp-content/uploads/[year]/[month]/ like the rest of time images and files that can be publicly available.

The files uploaded to this /wp-content/uploads/secure/ directory are being denied through .htaccess so that only PHP can access them, not the browser directly.

<Files *>
  Deny from all
</Files>

In theory.., not that hard. Especially since the file has a correct GUID in the database directly from the start. But after uploading a file and reloading the custom post type page, the above example no longer works without jumping through a whole lot of hoops.
I think we might have found a way by hooking into all the ajax hooks that are being fired for uploading, viewing and deleting a file to this custom post.

I'm not sure if we can show you a complete example of what we ended up with because it's so many hooks tied together. We'll let you know.

Hello Anh,

We did it with some action checks in the post. While the ajax-request runs it also runs the 'upload_dir' function. So we did it like the code underneed.

function change_upload_dir( $param ) {
    
    // Ajax request for deleting an uploaded file from MetaBox
    if( isset($_POST['action']) && $_POST['action'] == 'delete-post' ) {
        // Check if CPT is: Downloads
    }

    // Ajax request for the list of uploaded files from MetaBox
    elseif( isset($_POST['action']) && $_POST['action'] == 'query-attachments' ) {
        // Check if CPT is: Downloads
    }

    // Other actions
    else {
        //Check if post is cpt:bestanden
        if( isset($post->post_type) && $post->post_type == 'Downloads' ) {
        } 
    }
    
}
add_filter( 'upload_dir', 'change_upload_dir' );